Learn Finnish: Juhlimme syntymäpäivääni kuudentenatoista toukokuuta. - We celebrated my birthday on the sixteenth of May.

To say that something happens "on" a specific date, Finnish uses the essive case, which adds the ending -na or -nä. For teen numbers (11-19), only the first part of the word takes the case ending. The word for "sixth" (kuudes) changes to its essive form kuudentena, while the -toista part stays exactly the same.

Why is the month (toukokuu) in the partitive case (toukokuuta)?

When expressing dates, Finnish uses the partitive case for the month to mean "of May". The whole phrase translates literally as "on the sixteenth day of May".

Why is syntymäpäivääni (my birthday) in the partitive case?

The verb juhlia (to celebrate) is a partitive verb, meaning the thing you are celebrating must be in the partitive case. The word syntymäpäivä (birthday) becomes syntymäpäivää, and then the possessive suffix -ni (my) is attached to the end.

How do we know juhlimme means "we celebrated" in the past tense, rather than "we celebrate" right now?

For verbs ending in -ia or -iä like juhlia, the present and past tense forms are completely identical for the me (we) and te (you all) forms. This is because the verb stem already ends in an -i, which absorbs the past tense -i marker. You just have to rely on context to know if the event happened in the past or is happening now.