particle N5 essential casualpolitewritten

に — marking a point in time

に

Meaning

at / on / in — に attached to a specific clock time, day, date, or year to say when something happens

月曜日げつようびに試験しけんがある。

There's an exam on Monday.

Attaches to	Form	Example
Absolute time noun	N(time) + に	三時さんじに / 日曜日にちようびに / 春はるに

毎朝まいあさ六時ろくじに起おきる。

I get up at six every morning.

二千二十年にせんにじゅうねんに生うまれた。

I was born in 2020. (years take に)

週しゅうに三回さんかい運動うんどうする。

I exercise three times a week. (に = per a period)

Only absolute times take に. Relative time words (今日きょう, 明日あした, 来週らいしゅう, 毎日まいにち, 今いま) take no particle: ×明日あしたに, ○明日あした.
に also marks frequency 'per' a period: 一日いちにちに二回にかい (twice a day), 月つきに一度いちど (once a month).

Jengo shows に the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.

Sources Compiled with reference to A Dictionary of Basic Japanese Grammar.