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

Season 11 EU:

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