When is the first day of Sheva Brachos counted from?
The first day of Sheva Brachos is counted from the day that the Chupa took place. This applies even though the wedding meal will only take place that night, nevertheless the first day is counted as the day of the Chupa. Hence if the Chupa took place on Sunday prior to nightfall, then Sunday is considered the first day, and the seventh day is Shabbos. If the Chupa took place on Sunday after nightfall, then the first day of Sheva Brachos is counted from Monday and the seventh day is Sunday.
What part of the Chupa must take place before nighttime? The first day of Sheva Brachos is counted immediately after the completion of the Sheva Brachos that are recited under the Chupa. Hence if these Brachos were recited before nightfall, that day is considered the first day of Sheva Brachos. If the Brachos were recited after nightfall, then the night is considered the first day of Sheva Brachos even if the rest of the Chupa took place while day.
What is the law if the Sheva Brachos of after the Chupa took place during Bein Hashmashos?
Some Poskim rule we follow that day, and hence if the Chupa took place on Sunday by Bein Hashmashos, the first day of Sheva Brachos is counted from Sunday. Other Poskim however rule that in such a case the first day of Sheva Brachos is counted from Monday. Practically, Safek Brachos Lihakel, and one is to count from that day, Sunday.
 Rama 62/6; Rosh Klal 60
 Rama ibid
 Chelkas Mechokek 62/7; Derech Hachaim; Pischeiy Teshuvah 62/12; Olas Shmuel 4; Aruch Hashulchan 62/31; Kaf Hachaim 131/69
Other opinions: Some Poskim rule that the seven days begin starting from the day that the wedding meal took place. Accordingly if the Chupa took place before nighttime, but the wedding meal took place at night then that night is the first night of Sheva Brachos. [Kneses Yechezkal 60, brought in Pischeiy Teshuvah ibid; Shulchan Hatahor 131/16; See Shaareiy Teshuvah 131/14, however see Pischeiy Teshuvah ibid that states the word “no” is missing]
 Rama ibid
 Birchas Habayis 62/25; Nitei Gavriel 109/6
 The reason: As Bein Hashamshos is questionably day or night and hence Safek Brachos Lihgakel. [ibid]
 Siddur Beis Oved; Yad Ahron; Ohalei Yeshurun p. 32 in name of Rav Moshe Feinstein