Because the sum of dates is a multiple of 63, the time of the middle day can only be:

63÷7 = 9, or 63×2÷7 = 18, or 63×3÷7 = 27.

So it is possible to leave on May 6th, or May 15, or May 24th.

Departure on May 24th needs to be checked: the last day is 24+7- 1=30, and there is 3 1 day in May, which meets the conditions.

If you don't understand, please adopt it if you are satisfied.