A wedding is considered one of the happiest and meaningful occasions in Jewish Life. For this reason, Jewish custom restricts the dates when a wedding takes place.
1- First of all, since a marriage creates financial responsibilities and obligations, a wedding may not take place on the Sabbath or Jewish Holidays when work and financial dealings are not permitted.
2- Also, since a wedding is supposed to be an unrivaled event, Rabbinic law forbids marriage during the middle days of Passover or Succos, since the two happy occasions (Holiday and wedding) would be mingled.
3- There are also two extended periods during the Jewish Calendar year when tragedies of tremendous significance occurred to the Jewish People. During these weeks, no weddings are performed.
These times are:
a) Five out of the seven weeks between Passover and Shavuos, excepting Lag B'Omer. During these weeks, we commemorate the wiping out of Jewish communities during the Crusades as well as the deaths of thousands of Jewish Rabbinical leaders during Roman times. Different communities have differing customs regarding whether the first five weeks or the last five weeks are wedding-less. However, according to all communities, the weeks where they overlap are out.
b) The three weeks leading up to Tisha B'Av as well as the day after, commemorate the destructions of both of the Jewish Temples in Jerusalem as well as most subsequent tragedies of Jewish History.