"Come Sunday" is a phrase that generally refers to an upcoming Sunday. If you are looking for synonyms for this phrase, you could use "forthcoming Sunday", "upcoming Sunday", "next Sunday", or "this Sunday coming." Words like "approaching" or "impending" could also be used to imply the gathering or arrival of the upcoming weekend. Other alternatives to "come Sunday" could be phrases such as "come this weekend", "on Sunday", or "when Sunday arrives." Whatever your preference may be, these synonyms should prove to be quite useful when referring to the Sunday ahead.