Here's a general timeline of important dates. All official deadline dates are announced separately.
April
Playoffs end - offseason begins
Calendar year rollover
History/Award threads updated
Trading opens
Flex waivers open
Offseason extension period begins
May - June
Offseason extension period
July
Extension period ends
Keeper selection
August
Supplemental Draft
September
Free agency opens
October - January
Regular season
February
Trade deadline
March
Awards voting
Regular season ends
Free agency closes
Flex waivers close
Playoffs begin