To find out how many parties the skate park should have per day, you can divide the total number of parties by the number of days in the summer.
Total parties = 390
Total days = 65
Now, divide the total parties by the total days:
\[ \text{Parties per day} = \frac{390 \text{ parties}}{65 \text{ days}} = 6 \]
So, the skate park should have 6 parties per day.