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.