Hey there!
How to stop changing seasons when server has no online?

Pretty sure Fabric Seasons uses the world time to select the season, so if you want the season to not change when no one is online, you will have to make time stop in that case with a plugin on the server or something, and I don't know of one that only stops time. Alternatively you can make the seasons much longer, for example make them last an irl week per season

A quick search on Modrinth shows that there are multiple mods that do this, of which you can take your pick. They all should work about the same, but Ready Player Fun specifically states that it is designed for/supports seasons and weather.

Know one decision: install mod (don't remember the name) that will allow server processes like mobs, seasons do nothing when server has no online. But I want that only seasons do not process when server has no online. How can I do that?

Sorry for my English**