April 12 has Yuri's Night to commemorate Yuri Gagarin's first orbital flight by a human.
The launch of Sputnik 1, the first satellite humanity put into orbit, on 4 October 1957 at 10:58pm Moscow time is at least as important event, but I'm unaware of any organized events to commemorate the occasion.
In any case, Happy Sputnik Day! (It’s a national holiday for those of us who are citizens of the #L5Nation, celebrate it appropriately!)
This is picture of the day # 4727 on PhotoByFred.com: