ужинать = to dine / to have dinner... "I am dining at eight today" should be accepted.
Admittedly, the verb 'to dine' isn't used so much these days, or it has class connotations... but it's still good classic English usage.
Also, in British English 'dinner' is very often used to mean 'lunch'.
I've added it. There is nothing wrong with it aside from the fact that this is how proper English was spoken half a century ago. These days, it either dates you or else it rubs in your class and or elite education. (Nothing wrong with the latter, in my book -- better than a complete lack thereof.)