A client sent me a message this morning asking for a change to the software I'm developing.

It's Sunday, right?

Just don't answer? It probably can wait till tomorrow?

Well, it's not so easy with this client :'(

It's simple. You don't see the message. It's Sunday. You don't want to do it. He'll ask again another Sunday if you do.

Like everyone said, man, it's Sunday, every now and then comes a client sending messages about these things for me, and I simply don't see them, in fact, I really don't see them, having a separate number for work is mandatory for me.

"You will receive time and cost estimate by tomorrow noon"

Condition them to not contact you on weekends.
You have a life outside of their whims.