Japanese~when to add に after day of the week?

Question:

which one is correct
八月の第一金曜日シロメイでフォーマルがありました
八月の第一金曜日にシロメイでフォーマルがありました

Answer:

You put に after times that are specific. If you can point to it on a calendar or a clock, (Friday, August 14th, 5:00, etc.), then you use に. So the second one is correct.
For non-specific times, (tomorrow, next week, last year, etc.), don't use it.