When you have a rest day you have a holiday.
This is a jukugo word, which usually means on'yomi readings from the kanji. Since 日 has two on'yomi readings, here's a mnemonic to help you remember which one to use:
Every holiday, you eat not one, but two cucumbers. Gee, two (きゅうじつ)? Take it easy!
I drink wine on holidays.
Today is a holiday, so I came for a picnic.
Please do not email me on my days off.