nichiyoobi ni ikimasu.

kugatsu ni kaerimasu.

You do not need to use the time particle に with
1) time expressions defined relative to the present moment, such as "today" and "tomorrow",
2) expressions describing regular intervals such as "every day",
and 3) the word for "when".

You normally do not use ni with
1) the parts of day, like "in the morning" and "at night"
2) the word for "weekend".

Unlike words like ashita and maiban above, however,
these words are sometimes followed by ni , depending on style, emphases and personal preferences.

I read the newspaper in the morning.

asa (ni) shinbun o yomimasu.