When an action happens at a certain time, that time can be stated with a time argument. Time arguments come in two main forms:
Numerical time arguments
The time stated as we have just learned in this lesson can be turned into a time argument by adding the particle に
七時に起きます。
I get up at seven.
Non-numerical time arguments
Non-numerical time expressions can be used as arguments without any particle.
あした勉強します。
I’m going to study tomorrow.
However, with days of the week, the use of に is optional.
月曜日［に］行きます。
I’m going on Monday.
Numerical and non-numerical arguments can be combined.
きのう六時に起きました。
Yesterday I got up at six.