The current fixed calendar works in such a way so that the first day of Rosh Hashana can never fall out on a Friday, but this is the very day on which the first Rosh Hashana fell! What gives?
The Talmud in Rosh Hashanah (20A) explains that Yom Kippur shouldn't fall out on the day before or after Shabbat, since two consecutive Shabbat-like days could be problematic. If Rosh Hashanah were to fall out on Friday, Yom Kippur would fall out on sunday. Two reasons are given (see the gemara there for a discussion of these two reasons):
I would argue that these reasons weren't really applicable the day man was created, since there was no death in the world yet (until Adam sinned).
It would be interesting to see what Adam's observance of Yom Kippur would have been like, had he not sinned. Would he even fast? Would it still be day of Atonement?