I don't like the fact that the calendar is expecting spring as the first season, is there a way I can change the rotation so that the calendar matches summer > fall > winter > spring? Sorry!

Yes, you can open the calendar up in SimPE and relink all 20 days (pictures, and the matching holidays) into your preferred order. It's significantly much less work to just change your season rotation though. Why do you start your year in the summer?

It's the default order. I'm just fussy when it comes to secret sub-neighbourhood such as 'Hobbies' and Magical World, you cannot just change their season order and I don't know a way to work around with it and it feels weird if your season is spring and you go to Hobbies and it's set in summer. I have tried relinking all 20 days by changing literal number in BHAV but it didn't work when I tested it in game, the calender shows fifth day of spring picture when it's on fifth day of summer.

I wonder if there is a way of changing the hobby hood's season order within Simpe, it theoretically should be possible.

It's only the day picture you need to change, the season icon should be correct because it pulls that information from the hood's setting. I'd just recommend exporting all the images and reimporting them all onto the other day.

Oh okay, and what should I do about festival day notifications such as Lover's day?

You'll have to move them forward as well, or use the calendar without the notifications.

Okay. Thank you for answering! :D