The Shulchan Aruch (493) says that although the Omer should really be a happy time, it is the practice not to conduct weddings between Pesach & Shavuot until Lag B’Omer - because that is the period when the students of Rabbi Akiva died. The source for this is Yebamot 62, which says that 12,000 pairs of students (chavrutot?) died of askara (croup, or choking) because they did not treat one another with respect. The Gemara does not mention the semi-mourning practices, & some (e.g. Aruch HaShulchan) say that these practices only began in the time of the Geonim, in response to the bloody Crusades which decimated Jewish communities during this same Pesach-Shavuot time period. Some historians suggest that the students may have been part of Rabbi Akiva & Bar Kochba’s ill-fated rebellion against Rome, & this is what led to their deaths. Some traditions allow weddings from Lag B’Omer; others wait until 3 days before Shavuot (sh’loshet y’mai hag’bala).