Words that count days use an exceptional reading. The readings for 二十 are exceptions, and 日 takes the kun'yomi reading. Here's a mnemonic to help with that:
If you make one hat for twenty days you'll have enough hats (はつ) to fill up your car (か) completely! That's a lot of days and a lot of hats.
There is a test on the twentieth.
It will be 2019 in twenty days.
Payday is the twentieth day of every month.