に (time)
at, on, in (a specific time)
Time + に
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に after a time word says when something happens: 七時に起きます, "I get up at seven"; 日曜日に会いましょう, "let's meet on Sunday"; 八月に, "in August".
The catch is that only specific times take に: clock times, dates, months, years, days of the week. Words that are already measured from now stand alone: 今日, 明日, 昨日, 今週, 来年, 毎日 and 今. So it's 明日行きます, never 明日に行きます.
Days of the week sit in between: 日曜日に and plain 日曜日 are both heard, but に is the safer choice.
Examples
毎朝六時に起きます。
I get up at six every morning.
日曜日に映画を見ましょう。
Let's see a film on Sunday.
授業は九時に始まります。
Class starts at nine.
八月に日本へ行きます。
I'm going to Japan in August.
昨日は夜十一時に寝ました。
Last night I went to bed at eleven.