In this form “に”is used as a time marker to indicate the point of time an action will/has taken place.
3b. 7 時(じ) に 起(お) きます。 I get up at 7 o’clock.
2b. あしたテニスをします。 I will play tennis tomorrow.
3b. 映画(えいが) を 見(み) ました。 I watched a movie.
Such words as 「夏休(なつやす)み」or「誕生び日(たんじょう)」 can be followed by「に」.
「に」cannot be preceded by words like「今」「きのう」「きょう」「あした」or the interrogatives 「いつ」.
There are cases when the days of the week are sometimes not followed by「に」.