Toss up between Apollo 11 and Sputnik 1
In 1994 the US government made National Apollo Anniversary Observance week:
http://thomas.loc.gov/cgi-bin/query/z?r103:S24JN4-594:
Until the day comes that we are all one big united family as a species, I suspect that the US and Russia will celebrate their own space history anniversaries.
Of course who says there has to be just one space anniversary for the whole year?