Refers to the previous night or evening. Often used adverbially without a particle.
I couldn't sleep well last night.
The party last night was fun.
Around 11pm last night, a dog was barking in the neighborhood.
My fever hasn't gone down since last night.
There was an accident in front of the station last night.
Kana spelling is common in everyday writing, especially when the kanji reading might be ambiguous.
Rarely used kanji form; same jukujikun reading.