Question: I know that during the "three weeks" (between the 17th of Tammuz and the 9th of Av) one is not permitted to take part in a "joyous occasion". What is the defiition of such an occasion?

Answer: Firstly, concerning the period of the "three weeks," there is a difference in custom between Ashkenazim and Sefaradim. For Sefaradim the custom of mourning applies only from Rosh Hodesh Av, while for Ashkenazim it applies for the entire three weeks.

As to the definition of a "joyous occasion", there are different opinions. Some prohibit any type of music during this period, including television and radio. According to Arukh ha-Shulhan as well as the tradition of my Rabbi and teacher, Rav Soloveitchik, the prohibition concerns any gathering of 3 or more men or women (who are not family members) for a joyous, social gathering that includes food and/or music. Obviously, weddings, bar-mitzvahs, bat-mitzvahs are not held during this time. If the purpose of the gathering is for study or tzeddakah (communal needs), it is permitted.