Since this word is made up of a single kanji, it should use the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word:
It is the night, but it is your (よる) night.
I went to a party last night.
Last night, I was so excited that I barely slept.
Night is the time when monsters crawl up through the floor to have tea parties under my bed.