Time Reference
You need the particle に with the days of the week, numerical time expressions, and months. You do not need to use the time particle に with time expressions defined relative to the present moment, such as “today” and “tomorrow”, expressions describing regular intervals such as “every day”, or the word for “when.”
にちようびにいきます。 I will go on Sunday.
くがつにかえります。 I will go back in September.