I'm going to buy some envelopes at the night.
有任何疑问也欢迎提问,难道不是at night吗？要么也是in the night啊。the night也行，只是指定哪个晚上罢了。I'm going to buy some envelopes at night.我将在晚上去买一些信封。I'm going to buy some envelopes at the night.我将在那天晚上去买一些信封。