Working Sunday

Would I be totally unreasonable to refuse Da Boss's request that I
come in and work a full day this Sunday because I have an assignment
to write?

And is there anything they can possibly do to force the issue?

'Cos I really want to get my assignment done and I really don't want
to treck up here six days in a row, which is what they'll be asking
next week. Plus I'd still have to get in early on Monday *sigh*