Holiday
National Holiday, Public Holiday
noun, の adjective
A festival day literally means "festival day," as in a day for Shinto festivals or rituals. These days, though, it really just means holiday.
Usually this will be a national holiday or public holiday, since some Japanese public holidays are based on Shinto festival days. 祭日 was actually the official term for public holidays prior to World War II. It sounds a little old-fashioned today, but you may still encounter it occasionally.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
今日は祭日だからいつものカフェがお休みだったよ。
Today is a national holiday, so the coffee shop I usually go to was closed.
祭日なのに、仕事なの？
Do you have to work even though it's a national holiday?
祭日だってことを忘れていて、会社に来てしまった。
I forgot it was a holiday and I came to the office.