Who gets to pick first when summer vacations have to be scheduled for separated families? If you have a parenting agreement already in place then the simple answer is to follow the agreement. However, if you don't have a method for selection that's when the problems can begin.
My suggestion is that the parents alternate the priority in picking each year and that May 1st or May 15th be the date that the parent with priority must select if they want their first choice of weeks. So if the mother has priority in even years and the father has priority in odd years, then when May 1st or 15th comes if the parent with priority has selected their weeks then they get them. If the parent with priority has not picked then the other parent can pick first unless they delay and in the interim the first parent picks. So if Dad has priority and doesn't pick and on May 2nd Mom picks her two weeks, then Dad picks around those two weeks.
I also add that if the parent with priority picks late and therefore goes second, the other parent still goes first the following year as it is not their fault that the parent with priority did not go first.
So the key is to use your pick on time so that you don't have to pick second two years in a row.
Now for some parents, this will not matter as they are flexible but for a parent who can only take certain weeks off this can make a big difference.