Whitmonday vs Mon

Whitmonday and Mon both are nouns.

Word	Noun	Adjective	Verb	Adverb
Whitmonday	Yes	No	No	No
Mon	Yes	No	No	No

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

Whitmonday: 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 Whitmonday include Monday.

Whitmonday (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