Here are the major events for the theros block

Theros prerelease: September 21-22 - PASSED
Theros realease: September 27-29 - PASSED
Theros game day: October 20-21 - PASSED
Born of the gods prerelease: February 1-2
Born of the god realease: February 8-9
Born of the gods game day: TBA
Journey to nyx prerelease: TBA
Journey to nyx realease: TBA
Journey to nyx game day: TBA

Hope this helps!
I'll update as much as I can!