Learn Finnish: Tapaamme kahdentenakymmenentenäkuudentena kesäkuuta. - We will meet on the twenty-sixth of June.

This means 'on the 26th' and is the essive case of the ordinal number kahdeskymmeneskuudes (26th). In Finnish, every part of a compound number must inflect. So kahdes (2nd) becomes kahdentena, kymmenes (10th) becomes kymmenentenä, and kuudes (6th) becomes kuudentena.

Why does the essive ending switch from -na to -nä and back to -na inside that one word?

Even though compound numbers are written as a single long word, each part applies its own vowel harmony to its ending! Because of the vowels in the root words, kaksi gets the back-vowel ending -na, kymmenen gets the front-vowel ending -nä, and kuusi gets -na again.

Why is the month kesäkuu written as kesäkuuta?

When stating a date, the month is always in the partitive case. You can think of it as saying 'on the 26th of June'.

The verb tapaamme comes from tavata (to meet). Why does the v change to a p?