congratulatory (adj) – serving to express pleasure or joy for a person on a special occasion
BREAKDOWN: CON- (with) + GRAT- (favor) + -ATE (to make) + -ORY (serving to)
— to congratulate is to express joy at a special occasion
This word is built on the root GRAT-, which means thanks, favor, or pleasing.
Common GRAT- words include include agree, grateful, gratitude, greet, and greeting.
More esoteric GRAT- words include gratuitous.