～ に    

When we want to talk about the time when an action occurs, we add the particle 「に」 after the noun indicating the time. We use 「に」 for actions that occur in a short period of time. 「に」 is used when the noun indicates a specific time with a number attached, and it is not used when there is no accompanying number. However, in the case of days of the week, 「に」 can be used or omitted.