Yosef Karo

“ Laws that are customary during the time of the omer
דינים הנוהגים בימי העומר
493:1 It is customary not to marry a woman between Pesach and Shavuot until Lag BaOmer, because in that time, the students of Rabbi Akiva died. However, to do "erusin" and "kiddushin" (engagement and betrothal) is fine. ”

“ However, in many places, the custom is to cut their hair until Rosh Chodesh Iyyar, and they do not cut their hair from Lag BaOmer forward, even though it is permitted to cut one's hair on Lag BaOmer itself. But those places who have the custom to cut their hair from Lag BaOmer onwards, do not cut at all after Pesach until Lag BaOmer. And within one city, it should not be that some follow one custom, and some follow another, because of "lo titgodedu" ("do not cut yourselves" - Deut. 14:1, interpreted in Yevamot 13b as "lo te'asu agudot agudot," "Do not become seperate groups") ”