Whitsun Monday vs Mon

Whitsun Monday and Mon both are nouns.

Word	Noun	Adjective	Verb	Adverb
Whitsun Monday	Yes	No	No	No
Mon	Yes	No	No	No

As nouns, Mon is a hypernym of Whitsun Monday; that is, Mon is a word with a broader meaning than Whitsun Monday:

Whitsun Monday: the day after Whitsunday; a legal holiday in England and Wales and Ireland
Mon: the second day of the week; the first working day

Other hypernyms of Whitsun Monday include Monday.

Whitsun Monday (noun)	Mon (noun)
the day after Whitsunday; a legal holiday in England and Wales and Ireland	the Mon-Khmer language spoken by the Mon
	a member of a Buddhist people living in Myanmar and adjacent parts of Thailand
	the second day of the week; the first working day