What I got to know from youtube Yaz (summer) and kuş (winter) receives -in as a locative case while İlkbahar (spring) and Sonbahar (autumn) receive the regular locate case -de.
1) Long story short:
In Summer : Yazın
In winter : kışın
In spring : ilkbaharda
In autumn : sonbaharda
2) While to say on a day like on Monday, on Tuesday you use 'günü' Like
On Monday : pazartesi günü
On Tuesday: salı günü
...............And so on but it's optional to add günü as simply saying the day name can convey the meaning.
3) AND if you want to say something like on Tuesdays, on Fridays, you add -leri to the day name
On Tuesdays : salıları
4)And for saying in the morning, in the afternoon etc you CANadd the ending -leyin.
Sabahleyin = in the morning
Geceleyin = at night
But this is very optional as simply saying the part of the day can convey the meaning
I hope it was helpful
Ps: I am not a native Turkish speaker, correct me if I am wrong anywhere