What's the difference between 恭喜發財 and 恭禧發財?
The correct way to write the phrase is「恭喜發財」. This phrase is made up of two words:


「恭喜」, meaning congratulations;
「發財」, meaning to make a fortune.


In contrast,「恭禧」is not actually a word. At most,「恭禧發財」can be viewed as a pun which emphasises the fortune aspect of the phrase, as「禧」(good fortune) is a homophone with「喜」.