In the following sentence, what is most appropriate: in 1977 or of 1977?
For Oprah, April Fools' Day in 1977 wasn't funny at all.
Neither. April Fools' Day represents a date, and no preposition is required at all.
22 June 1980 was a good day.
For Oprah, Christmas Day 1979 brought many gifts.
For Oprah, April Fools' Day 1977 wasn't funny at all.
If you are discussing the year in toto and drawing attention to a particular day, then either in or of will work. In is probably more likely, but I suspect that a corpus search for this particular construction won't be particularly easy. However, in that case, the year will be understood and can be omitted altogether (and the preposition with it).