There is a test on the twentieth.
It will be 2019 in twenty days.
Payday is the twentieth day of every month.
Two times ten days is twenty days.
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.