1. Question: Why can you remarry a woman you divorced after Nisuin but not a woman you divorced after Kiddushin (Erusin) anytime during the omer? Background:
A. Weddings are forbidden during parts of the omer when we are mourning over the loss of Rebbe Akiva’s students because it creates a tremendous amount of joy.
B. There are two parts to a halachic marriage, Kiddushin which is the acquisition by giving the ring and the like and nisuin which is the consummation. Now a days we do them all at the same time but they use to be done separately like an engagement and a wedding.
C. A get (divorce bill) is required in both cases and a Kiddushin and nisuin must be redone both times too.
Answer: There is no halachic joy when remarrying your wife from Nisuin because you are just getting back together, nothing new. But to consummate a Kiddushin that was broken off is a new step to the marriage so it has a lot more joy. (See Dirshu Mishna Berura 493 footnote 2)