you can say "また明日" and "また日曜日（に/ね/にね）" for greeting. "また日曜日やん" does not mean "see you on sunday", might means "it's on sunday again!!" in a dialect...

The expression "See you tomorrow / on Sunday" actually has the meaning of "Let's meet tomorrow / on Sunday".　Hence, in this case, the Japanese translation is「また 明日に / 日曜日に 会いましょう。」　In short form, this becomes「また 明日に / 日曜日に」, and you may add「じゃね。」to indicate "bye".　That is it.　It is just that easy.　Good luck.