Mon vs Whitsun Monday

Mon and Whitsun Monday both are nouns.

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

As nouns, Whitsun Monday is a hyponym of Mon; that is, Whitsun Monday is a word with a more specific, narrower meaning than Mon:

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

Other hyponyms of Mon include Whitmonday.

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