31days用英语表达为"31 days". 来源是因为"31"表示一个月的天数，"days"表示天的单复数。下面是一些例句：
I'm going to do a 31-day challenge to improve my health.
我要挑战31天来改善我的健康。
I can't bepeve I've been working on this project for 31 days straight.
我简直不敢相信我已经连续工作了31天了。
I'm going to start a 31-day gratitude journal to practice being thankful.
我要开始写31天的感恩日记来练习感恩。