Mardi Gras vs Fat Tuesday

Mardi Gras and Fat Tuesday both are nouns.

Word	Noun	Adjective	Verb	Adverb
Mardi Gras	Yes	No	No	No
Fat Tuesday	Yes	No	No	No

As nouns, Mardi Gras and Fat Tuesday are synonyms defined as:

Mardi Gras and Fat Tuesday: a carnival held in some countries on Shrove Tuesday (the last day before Lent) but especially in New Orleans

As nouns, Mardi Gras and Fat Tuesday are synonyms defined as:

Other synonyms of Mardi Gras include pancake day, Shrove Tuesday.

Mardi Gras (noun)	Fat Tuesday (noun)
a carnival held in some countries on Shrove Tuesday (the last day before Lent) but especially in New Orleans	a carnival held in some countries on Shrove Tuesday (the last day before Lent) but especially in New Orleans
the last day before Lent	the last day before Lent