Spirit_of_Sky все записи автора The preposition on is used when expressing one’s good wishes in the context of a happy event:

Congratulations on your marriage!
Congratulations on your new baby!
Congratulations on your promotion!
Congratulations on winning the lottery!

Congratulations can be offered as praise for someone’s achievement. In that context, the preposition to use is for:

Congratulations for completing 100 days without an accident!
Congratulations for leading the Scouts to safety!
Congratulations for saving the farm from foreclosure!