Seasons SimGuru/Maxis-Sourced Info

Spring, Summer, Fall, Winter

https://twitter.com/EveningBirdx/status/1001939416515010560
https://twitter.com/SimGuruEmory/status/1001941665626644481

Season Defaults
https://twitter.com/SimGuruGrant/status/1001933473102282752
https://twitter.com/SimGuruEmory/status/100008791589914214
And in new saves?
https://twitter.com/_/status/1001520237924274176
From the livestream*:

• My favorite info bit from the stream is that the days have different length according to seasons, so the sun will set earlier in winter than in summer.
  

Seasonal Changes
https://twitter.com/_/status/1001875005511290885
https://twitter.com/BroBastii/status/1001935395079249920
https://twitter.com/SimGuruEmory/status/1001941163618783232
Kimmi made the new Seasons icons on the selector screen:
https://twitter.com/SimGuruEmory/status/1005874399705120768

Season Customization
https://twitter.com/SimGuruEmory/status/1000452880728403969
https://twitter.com/SimGuruEmory/status/1000171842772979714

https://twitter.com/matias11775369/status/1005468105399853056
https://twitter.com/SimGuruGeorge/status/1005478157015642114

https://twitter.com/SimGuruEmory/status/1006521104687493120
https://twitter.com/SimGuruEmory/status/1006695536554999808

Seasonal Activities

Seasonal Activities: Create a winter story with ice skating, befriend a magical scarecrow in fall, share a spring fling during a downpour, or host a summer pool party.- blog post

Bundle up to build a snow pal with friends, create snow angels or perform cool tricks at the skating rink. Toss on a raincoat to splash in a spring shower and explore the outdoors to earn badges as a scout. Dress Sims in new swimwear to cool off in the sprinkler or play in the kiddie pool. Harvest honey in the fall, play in leaves or have child Sims make homemade holiday crafts. - Origin

https://twitter.com/SimGuruKate/status/1001934568553725952

It wouldn't be fall and winter without a flurry of chillier activities. So, for the first time in The Sims 4, we're introducing ice skating. Tricks, spins, and awkward falls await your Sims when you place the skating rink. Plus, Sims can arm themselves with hats and gloves, then head outside to make a snowpal and snow angels.

Speaking of seasonal happenings, are your Sims ready to fall in love? Adventurous Sims can ahem express their feelings for each other in a leaf pile! Yeah, you'll probably want to leaf them alone for a while. - blog

https://twitter.com/SimGuruGeorge/status/1005323842502324224

https://twitter.com/AtariShooter/status/1007270892475441152
https://twitter.com/SimGuruMorgan/status/1007271442122076160

Re the skating rinks:
https://twitter.com/SimGuruGrant/status/1005097930598989825
https://twitter.com/mi_tlumbob/status/1005099774612267008
https://twitter.com/SimGuruGrant/status/1005100124182228994
https://twitter.com/AngelBubby1/status/1005620172072206337
https://twitter.com/SimGuruGrant/status/1005621174104027137

Seasonal decor:
https://twitter.com/SimGuruGeorge/status/1002356837272842240
https://twitter.com/SimGuruGeorge/status/1004808654405529600

https://twitter.com/oneorafael/status/1004781674855616515
https://twitter.com/SimGuruGeorge/status/1004782758780997634

https://twitter.com/SimGuruGeorge/status/1005949876117499905

The seasonal time doesn't include months:
https://twitter.com/SimGuruGrant/status/1005303180928811008

*Thanks to @Tomatplante for livestream info