Season 12 EU:

What	When
Season 12 [EU] Signups Open	1st of March (20:00 CET)
Season 12 [EU] Signups Close	5th of April (23:59 CEST)
Season 12 [EU] Seeding	6th of April
Season 12 [EU] Round 1 starts	20th of April
Season 12 [EU] Round 10 starts (last round)	22nd of June
Last date to play regular season games	3rd of July
Season 12 [EU] Draw Show	5th of July
Season 12 [EU] Playoff Group Stage starts	6th of July
Season 12 [EU] Rare Cup Finals	25th of July
Season 12 [EU] Epic Cup Finals	26th of July
Season 12 [EU] Legendary Cup Finals	1st of August
Season 12 [EU] Mythic Championship Finals	2nd of August
Season 12 [EU] Playoffs end	2nd of August

Season 4 NA:

What	When
Season 4 [NA] Signups Open	3rd of November
Season 4 [NA] Signups Close	9th of February
Season 4 [NA] Seeding	10th of February
Season 4 [NA] Round 1 starts	16th of February
Season 4 [NA] Season 4 [NA] Round 8 starts (last round)	12th of April
Last dat to submit replays	17th of April
Season 4 [NA] Draw Show	19th of April
Season 4 [NA] Playoff Group Stage starts	20th of April
Season 4 [NA] Epic Cup Group Stage	20th of April - 16th of May
Season 4 [NA] Epic Cup Finals	17th of May
Season 4 [NA] Legendary and Mythic Group Stage	20th of April - 22nd of May
Season 4 [NA] Legendary Cup Finals	23rd of May
Season 4 [NA] Mythic Championship Finals	24th of May
Playoff Ends	24th of May

What	When
Season 11 [EU] Signups Open	13th of October (20:00 CEST)
Season 11 [EU] Signups Close	17th of November (23:59 CEST)
Season 11 [EU] Seeding	18th of November
Season 11 [EU] Round 1 starts	2nd of December
Season 11 [EU] Round 3 starts	16th of December
Season 11 [EU] Christmas break	2 weeks of no matchmaking
Season 11 [EU] Round 4 starts	6th of January
Season 11 [EU] Round 10 starts (last round)	17th of February
Season 11 [EU] Last date to play regular season games	28th of February
Season 11 [EU] Draw Show	1st of March
Season 11 [EU] Playoff Group Stage starts	2nd of March
Season 11 [EU] Rare Cup Finals	21st of March
Season 11 [EU] Epic Cup Finals	22nd of March
Season 11 [EU] Legendary Cup Finals	28th of March
Season 11 [EU] Mythic Championship Finals	29th of March
Season 11 [EU] Playoffs end	29th of March