Complex sentence: Yuri's Night is an international celebration held on April 12 every year to commemorate the first human in space, Yuri Gagarin on April 12,1961, and STS-1, the first Space Shuttle launch on April 12,1981.
Split into simpler sentences: Yuri's Night is an international celebration held on April 12 every year to commemorate two separate space exploration milestones. The second milestone was the launch of STS-1, the first Space Shuttle launch on April 12,1981.