Overnight Stay, Sleepover, Stopover
noun
You know that 泊まる is "to stay the night." The noun version is a overnight stay or sleepover.
とまり
The reading comes from 泊まる, so pull from that and you shall know this.
うちの子、幼稚園のお泊まり会をすごく楽しみにしてたんだけど、風邪を引いちゃって、今抗生物質を飲ませたところなんです。だから、残念ながら今夜は欠席させて頂きます。
My kid was really looking forward to the kindergarten sleepover, but he got a cold and I just made him take antibiotics. So, sadly, he’ll be absent tonight.