In my observation, most people observe certain mourning customs between the beginning of sefirat ha'omer and lag b'omer (such as refraining from shaving, listening to live music, etc.). Some people continue to observe the mourning customs after lag b'omer, while others do not. What are the reasons for continuing to observe the mourning, and what are the reasons for stopping at lag b'omer?
As brought here, the two opinions are the Mechaber and the Ramo. The Mechaber holds that the proper time to morn the students of Rabbi Akiva is when it happened, whereas the Ramo holds that it should be on the days that Tachnurn is not said. The Arizal holds that the issue on Sefira is that they are days of judgement, which is why the students died then, so one should not get a hair cut the entire Sefira. The Rebbe Rashab (the fifth Lubavitcher Rebbe) includes the last three days before Shavuos. Whether or not this applies to weddings, etc. is a matter of dispute.
Minchas Elazar (Chelek 4, Siman 44, 60) holds that it only applies to hair cuts, and that in fact getting married, especially on Lag BeOmer is a good thing.
The Divrei Yoel disagrees, and forbids weddings as well.
The Lubavitcher Rebbe writes that it is preferable to hold the wedding after Shavuos, but if the wedding is specifically desired on Lag BeOmer, it should be during the day of Lag BeOmer and not the night of Lag BeOmer.