Season 13 EU:

What
When
Season 13 [EU] Signups Open
5th of July (20:00 CEST)
Season 13 [EU] Signups Close
9th of August (23:59 CEST)
Season 13 [EU] Seeding
10th of August
Season 13 [EU] Round 1 starts
24th of August
Season 13 [EU] Round 10 starts (last round)
26th of October
Last date to play regular season games
6th of November
Season 13 [EU] Draw Show
6th of November
Season 13 [EU] Playoff Group Stage starts
9th of November
Season 13 [EU] Rare Cup Finals
28th of November
Season 13 [EU] Epic Cup Finals
29th of November
Season 13 [EU] Legendary Cup Finals
5th of December
Season 13 [EU] Mythic Championship Finals
6th of December
Season 13 [EU] Playoffs end
6th of December

Season 5 NA:

What
When
Season 5 [NA] Signups Open
19th of April
Season 5 [NA] Signups Close
21st of June
Season 5 [NA] Seeding
22nd of June
Season 5 [NA] Round 1 starts
6th of July
Season 5 [NA] Round 8 starts (last round)
24th of August
Last day to submit replays
4th of September
Season 5 [NA] Draw Show
6th of September
Season 5 [NA] Playoff Group Stage starts
7th of September
Season 5 [NA] Epic Cup Group Stage
7th of September - 3rd of October
Season 5 [NA] Epic Cup Finals
4th of October
Season 5 [NA] Legendary and Mythic Group Stage
7th of September - 9th of October
Season 5 [NA] Legendary Cup Finals
10th of October
Season 5 [NA] Season 5 [NA] Mythic Championship Finals
11th of October
Season 5 [NA] Playoff Ends
11th of October

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