Here's your quote:
Each month, the school holds a party. This event is / These events are/ successful because it attracts / they attract over two hundred students.
The issue is to determine what the subject of the second sentence (this event / these events) refers to.
The supplied context neither identifies a specific party nor constrains the interpretation to a (single) generic party. There is no singular party to use for numerical agreement.
It would be natural for the subject of the second sentence to refer to the set of monthly parties (plural).
Each month, the school holds a party. These events are successful because they attract over two hundred students.
This holds regardless of the frequency of the parties. They could be a thousand years apart, and the natural reading would still be for plural agreement.