–   Q. See question above. Can one celebrate a Siyum on that day or should preferably postpone it.

A. Shulchan Aruch O.H. (580: 2) enumerates a list of days that were identified as days of mourning due to tragedies and disasters that occurred on them in our history, and it is proper when possible to declare them a fasting day.

Shulchan Aruch mentions the 18th day of Av as the day when the Eternal Light on the Beth Hamikdash was extinguished . Mishna Berura (ibid 6) quotes opinions that it happened on the 17th and one should mourn on both occasions.

However, a Siyum in the honor of the Kedoshim is proper and recommended.

Rabbi A. Bartfeld as revised by, Horav Yaakov Hirschman, Horav Dovid Pam, Horav Aharon Miller, Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a