146:1 The practice is for a groom and bride (even a widower and a widow – Mishnah Brurah 573:8) to fast on their wedding day because on this day they are forgiven for their sins. At mincha, they say “Aneinu” as on other fasts. (If they already drank from the cup at the chupah, they may no longer say Aneinu – MB 562:12.) The practice in our countries is only to fast until after the ceremony; if the ceremony is delayed until night, i.e., after the stars come out, they may eat something, but they may not yet drink any alcoholic beverages.
146:2 A bride and groom do not fast on Rosh Chodesh, on the day after Shavuos, on the 15th of Av, on the 15th of Shevat, on Chanukah or on Shushan Purim. During the month of Nisan, even on Rosh Chodesh Nisan, they do fast, as well as on the Lag b’Omer, the days between Rosh Chodesh Sivan and Shavuos, and the days between Yom Kippur and Succos. (One would also fast on Rosh Chodesh Av, though one should optimally not marry on that day – MB 573:9.)