Dear Rabbi In some shuls I attend, which daven nusach sefad, on Shabbos/rosh chodesh/chagaim, they recite aleinu after shacharit and musaf. Can you explain the reason for this?
Shalom, Thank you for your question. Firstly let's try to understand the reason why we say Alainu at the end of most prayer services. The Bach (Orach Haim 133) explains that the custom to say Alainu at the end of the service is in order to fix in our hearts the belief in Hashem before we leave shule and head homewards. Based on this we can understand the general practice not to say Alainu at the end of Shacharit on a day when we are going to immediately say Musaf – if we have finished Shacharit but are not leaving Shule then we have no reason to say Alainu until after Musaf just before we head on home. (In fact some Shules apparently had the custom not to say Alainu after Mincha when they stayed in Shule to immediately pray Maariv [see Mishna Brurah 132,7] – although I have never seen this custom in practice). The Bach also writes that we say Alainu to strengthen our belief in Hashem before going out to do business in a non-Jewish marketplace, which requires extra care so as not to be swayed by the idol worship one might meet. Based on this there was perhaps a custom at one time to only say Alain on a weekday service before going out to work – see the response of the Kol Mevaser (volume 2, 31). This all explains why the vast majority of Shules do not say Alainu after Shacharit when they are going to say Musaf. However, you are correct that there are a few places that do say Alainu after Shacharit and Musaf. The work Piskay Tshuvot (132,6) explains that this is based on an understanding from the Ari HaKadosh that Alainu has kabalistic spiritual properties that are particularly needed at the end of each of the daily prayers. As such it should be said also at the Shacharit. The halacha is that we do not say Alainu after Shacharit if it is to be followed by Musaf – unless this is the custom of the Shule (see Kol Mevaser ibid). Blessings.