Per Matt, always can be a change but generally its this.
Normal events
February: Valentine's event for 2 weeks (with lobby 3)
April: Easter event for 4 weeks (with lobby 4)
August: Anniversary event for 4 weeks (with lobby 15)
October: Halloween event for 2 weeks (with lobby 5)
December: Christmas event for 4 weeks (with lobby 1)
Time-limited quests
February: Valentine's Day-124 (during Valentine's event)
March: White Day-125 (2 weeks period, same as Valentine's but in March) and Principal's Gift-220 (entire month)
August: Maximum Attack E-930 to 940, Festivity on the Beach-232 (during Anniversary event)
October: Pioneer Halloween-207 (during event)
December: Pioneer Christmas-240 (during event)
Lobbies
January: New Year's-7 (until Jan 16 after Christmas)
February: Valentine's-3 (during event)
March: White Day-9 (during March, same days as Valentine's)
April: Easter-4 (during event)
May: Cherry Blossom-8 (2 weeks after Easter)
June: Wedding (3 weeks during June)
August: Anniversary-15 (during event)
September: Autumn-11 (21st September until Halloween event)
October: Halloween-5 (during event)
December: Christmas-1 (during event)
