When you compliment someone, you could be complementing his or her day. This is actually a hard one for me to remember and another one letter voyage to misunderstanding if it’s wrong.

Compliment - noun or verb - used for praise, courtesy, esteem, affection, admiration.

Complement - noun or verb - something that fills up, completes, or supplements

One way I use to remember which vowel to use is that complement has the “e” as in complete. And compliment has an “i” as in I like. Easy as pie.