日曜 is the abbriviation for 日曜日 and there is no difference in meaning.
However, because 日曜 is the abbriciated form you will find it more during places such as conversations and advertizements. You will find 日曜日 more in books or formal documents (not saying 日曜 is informal).
You can also use 日曜日 when discriving general things and 日曜 for the a particular date.
例:「私は毎週日曜日はプールに行きます。」I go swimming every Sunday.
「次の日曜空いてる?」Are you free next Sunday? (conversational)