Question: [Wednesday, 14th Marcheshvan, 5782]
I forgot to say or hear Havdalah on Saturday night and just remembered to do so now on Tuesday night? May I still say it?
No. It may only be said up until the start of Tuesday evening, until sunset of Tuesday, and not any further.
Explanation: The main Mitzvah of Havdalah is to say it at night on Motzei Shabbos. If one did not do so, then according to the Ashkenazi ruling, he may say Havdalah up until Tuesday, including Tuesday, as the first three days of the week are considered the days after Shabbos, and are included in Motzei Shabbos. However, starting from Wednesday, he may no longer say Havdalah, as the last three days are considered the days before the coming Shabbos and have no connection to the previous Shabbos. Now, according to the Jewish calendar the start of Wednesday begins from nightfall on Tuesday, and one is to be stringent starting from sunset. Hence, Havdalah may only be recited up until Tuesday night and from the start of Tuesday night, starting from sunset of Tuesday, it may no longer be said.
Sources: See Admur 299:8 “he is to do so on Tuesday up until the end of the day, however, from that time and onwards he can no longer say it.”; Michaber and Rama 299:6; Pesachim 106a; Ben Ish Chaiy Vayeitzei 23; Kaf Hachaim 299:26; Makor Chaim of Chavos Yair 299, brought in Piskeiy Teshuvos 299:10, that may not say past sunset of Tuesday