Ascension Day vs Ascension

Ascension Day and Ascension both are nouns.

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

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

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

Other synonyms of Ascension Day include Ascension of the Lord.

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