Question: I have been invited to a chasuna during the omer. Can I attend even if I am observing that 'half', and can I shave?
Answer: The Shulchan Aruch (OC 493:1) writes that R’ Akiva’s students died during the omer (See Yevamos 62b). Therefore, we observe certain mourning practices during the omer, including no haircuts.
There are different minhagim as to which ’half of the Sefira’ to observe. The Shulchan Aruch (OC 493:2) writes that one should observe from the beginning until after Lag B’Omer, while the Rema allows one to take a haircut on Lag B’Omer. Others observe the ‘second half’, from Rosh Chodesh Iyar until Shavuos (See Rema OC 493:3; Magen Avraham 493:5). The Mishna Berura (493:14; Biur Halacha 493:3) explains that while there are different reasons for each of these minhagim¸ regardless, everybody observes these mourning practices for thirty-three days. However, some avoid taking haircuts throughout the omer except on erev Shavuos (See Shaarei Teshuva 493:8; Kaf Hachaim OC 493:13).
R' Yitzchak Yaakov Weiss (Minchas Yitzchak 4:84) writes that one observing ‘one half’ of the omer can attend a chuppa during that half, but should not participate in the chasuna.
However, R’ Yaakov Kamenetsky (Emes L’yaakov OC 493:n465), R’ Moshe Feinstein (Igros Moshe OC 1:159; 2:95) and R’ Shlomo Zalman Auerbach (Pesach 11:19) allow one to attend the chasuna and participate normally. As it is permissible for the chassan and kalla, everyone may participate.
However, R’ Moshe Feinstein writes, that one attending a wedding may only shave if he would be too embarrassed to go unshaven. Had he been invited before the omer, he should have rather kept the other half so as not to have to rely on this leniency.
In conclusion, one may attend a wedding even while observing the omer, though one should not ordinarily shave. One who is embarrassed to go unshaven may do so, if necessary, though had they known in advance, they should have chosen to observe the ‘other half’ of the omer.