### Instruction: If the first day of a month is Monday, what day of the week is the twenty-third day?### Response: I know that a week has seven days, so I can use modular arithmetic to find the day of the week of any day in a month.
If the first day is Monday, then the eighth day is also Monday, the fifteenth day is also Monday, and the twenty-second day is also Monday.
Therefore, the twenty-third day is one day after Monday, which is Tuesday.