Names of idols-Saying the names of the weekdays, secular months, and planets which are named after idols:
A. The Halachic background:
The prohibition against mentioning the name of an idol: It is forbidden to mention the name of an idol or foreign deity. This applies whether there is a need to mention his name, such as to tell his friend to wait for him by a certain idol, and whether there is no need to do so, and one wants to mention his name in casual conversation.
Names of idols written in Scripture: It is permitted to mention the names of idols written in the Torah, such as Kara, Baal, Koreis, Nevo, Haorchim Lagad Shulchan, [Baal Tzafon]. [The reason for this is because these idols have become extinct and nullified, and it is only prohibited to mention the name of idols that are still in existence, and are still worshiped. Alternatively, the reason that it is permitted to mention the names of idols written in Scripture is because since the Torah mentions it, therefore certainly we may mention it.]
Names of idols that do not connote a deity in their language: This above prohibition only applies to names of idols and deities that were innovated for the sake of the idolatry, and hence the meaning of the name itself connotes a foreign god [such as Zeus, which connotes Deity in Greek]. However, common names of people/items which have been used for idols, may be mentioned, as the name does not have any godly connotation, and was not innovated for this purpose. Thus, we find that the Gemara mentions the names of festivals of idolatry, as well as the name of Oso Ish and his students.
B. The history of the names of the weeks, months, and planets:
Some of the names of the weekdays, secular months [i.e. January, etc.], and planets, were given in association with worship of foreign deities in the pagan religion.
The weekdays: While most of the weekdays were named after deities, some were not. For example, Tuesday is named after the deity called Tiu, a deity of the Teutonic mythology. What is the was named after their deity called Woden. Thursday was named after their deity called Thor. Friday was named after Frig, the wife of Odin, another deity in this mythology. Nonetheless several weekdays were not named after the names of a deity, such as Sunday which was named after the sun, Monday which was named after the moon.
The secular months: The vast majority of the secular months were not named after deities but after kings and seasons and Latin numbers. Nonetheless, some of the months are named after deities such as January and March. Thus, for example, the month January was named after the Roman deity named Janice. the month of February was named after a holiday that they kept, the month of March after a Roman god, the month of April after the blossoming season, the month of May after the goddess Maias, the month of June after the Roman goddess Juno, the month of July after the Roman dictator Julius Caesar, the month of August after the first Roman Emperor Augustus, the month of September after the number seven in Latin being that it is the seventh month according to the old Roman calendar, the month of October after the number eight in Latin, the month of November after the number nine Latin, and the month of December after the number 10 in Latin.
The planets: The five planets of Mars, Mercury, Saturn, Venus, and Jupiter, are attributed to Roman mythology being named after the names of their deities. Uranus and Neptune which were discovered only the 1600s were not named after deities. Pluto was only named in the 1900s, but was named after a Roman deity. The name earth does not come from Roman mythology but rather from old English.
C. The law:
It is permitted to mention the names of the weekdays, secular months, and planets, as the deities for whom they are named after are extinct and no longer worshiped, and hence follow the same allowance as mentioning the names of idols brought in Scripture [and as saying the name Tamuz for the month of Tamuz]. Furthermore, many of the names do not connote a deity in any language, and some are even names of items, such as the sun and moon [i.e. Sunday and Monday] and hence also follow the allowance to mention the name of deities that do not connote a foreign God in their translation. Thus, we find even amongst the early Poskim those who would use the secular names of the months. Nonetheless, some God-fearing Jews are stringent not to mention or write the names of the secular months, or do not write them in full, and seemingly their stringency would likewise apply to the planets and some of the weekdays [i.e. Tuesday-Saturday]. Practically, the Rebbe writes that the custom of Jewry is not to be careful against reciting the secular names of the week, and in practice our Rabbeim would write the names of the secular months in their letters and were not stringent in this matter at all.
An additional issue with the weekdays: An additional halachic issue that is raised regarding the secular names of the weekdays is that perhaps it transgresses the command to count the days to the Shabbos, and hence Yom Rishon, Yom Sheiyni, etc. Practically, the custom of Jewry is not to be careful in this matter, as in truth there is no explicit command to always refer to the weekdays in a way that commemorates the Shabbos.
 Michaber Y.D. 147:1; Sanhedrin 63b
 The source: This is learned from the verse “Vesheim Elokim Acheirim Lo Sazkiru, Lo Yishama Al Picha”
 Michaber ibid; See Taz 147:1 for the novelty of this ruling
Other opinions: Some Poskim rule that when there is an absolute necessity one may mention the name. [Chavos Yair Teshuvah 1 Hasaga 11-12]
 Michaber 147:4; Sanhedrin 63b
 Shach 147:2; Perisha
 Yireim 245 ; See Tofas Riem for different Girsas and if these are two reasons or one; Levush 147:4; See Chavos Yair Teshuvah 1 Hasaga 11-12 who questions the first reason; See Likkutei Sichos 23:166 that the Torah’s mentioning of the name destroys it and makes the idol worthless
 Yireim 245  “If they were given a name that connotes a deity”, brought in Hagahos Maimanis Avodas Kochavim 5:3; Beis Yosef 147:2 in name of Rabbeinu Yerucham in name of Avi Ezri [censored from some editions of Tur] “Specifically if the name is a name of sovereignty and deity”; Chavos Yair Teshuvah 1 Hasaga 11-12; Biur Hagr”a 147:3; Teshuvos Rav Azriel Hildsheimer 180
 The reason: As only those names that were innovated for the sake of idolatry was the Torah particular against one mentioning. [Poskim ibid] See Chavos Yair ibid that according to Yireim ibid even names that were innovated for a deity may be used if they do not connote a meaning of a deity in the language. However, Chavos Yair questions this and asserts that perhaps only names that were already used for other purposes have this allowance.
 Poskim ibid; See Sanhedrin 43; 67; 105; 107; Avoda Zara 27; Yerushalmi Brachos 5:1
 Britannica encyclopedia
 So rule regarding the months, and the same would apply regarding the weekdays and planets: Iyun Yaakov Megillah 2 Pirush Hakosev; Yabia Omer 3:9; Piskeiy Teshuvos 156:3
 This follows the first reason of the Yireim ibid; see previous footnotes for a discussion of this reason and those who question or negate it
 See Beis Yosef 117; Abudarham p. 110 [there he mentions the months of November, February, and October]; Kitzur SHU”A 19:5 [writes December]; Koveitz Beis Aaron Veyisrael 68 pages 109-110
 See Tzitz Eliezer 8:8 that one is to avoid doing so being that these names are rooted in idolatry. It is unclear if they are also stringent not to mention the weekdays, or if they in any event only mention it in Hebrew, such as Yom Rishon, Sheiyni etc
 Piskeiy Teshuvos 156:3 footnote 17
 Igros Kodesh 2:361 [printed in Shulchan Menachem 1:93]; Divrei Yoel O.C. 15; Piskeiy Teshuvos 156 footnote 18
 See Igros Kodesh 2:361 [printed in Shulchan Menachem 1:93]; Divrei Yoel O.C. 15; Piskeiy Teshuvos 156 footnote 18
 See Ramban Shemos 20:8 “The Jewish people count all the days to the Shabbos, “the first of the Shabbos the second of the Shabbos etc”…” The Gentiles count the days of the week for the day itself by a different name or by the name of one of their saints such as the Christians, or other names. However, the Jewish people count all the days for the sake of Shabbos, as this is the command that we were given to always remember the Shabbos”; Mechilta Yisro 8 “Do not count like others count but rather count for the day of Shabbos”; Sefer Chareidim Mitzvos Hateluyos Bapeh 13:2 in name of Ramban
 Igros Kodesh ibid; Divrei Yoel ibid; Igros Kodesh 2:361 [printed in Shulchan Menachem 1:93]
The reason: 1) As even according to the Ramban, it is obvious that the mitzvah does not obligate one to mention the Shabbos every time he mentions the weekday and rather if he mentions it one time that day it suffices. Hence, when we say the Shir Shel Yom in Shachris, we already fulfill our obligation. 2) Even initially, we do not follow the opinion of the Ramban, as in his opinion it would be forbidden to ever say the weekday without mentioning Shabbos, and I’ve never seen anyone be careful against writing or saying Sunday or Monday.