Ascension vs Ascension Day

Ascension and Ascension Day both are nouns.

Word	Noun	Adjective	Verb	Adverb
Ascension	Yes	No	No	No
Ascension Day	Yes	No	No	No

As nouns, Ascension and Ascension Day are synonyms defined as:

Ascension and Ascension Day: (Christianity) celebration of the Ascension of Christ into heaven; observed on the 40th day after Easter

Other synonyms of Ascension include Ascension of the Lord.

Ascension (noun)	Ascension Day (noun)
(New Testament) the rising of the body of Jesus into heaven on the 40th day after his Resurrection	(Christianity) celebration of the Ascension of Christ into heaven; observed on the 40th day after Easter
(Christianity) celebration of the Ascension of Christ into heaven; observed on the 40th day after Easter