Depending on what type of disagreement arises, it could potentially mean both. Consulting with an attorney in a confidential setting is the best way to analyze an Order's language with due diligence.
It means that in different years one parent then the other gets 1st pick by a date certain. Then the other parent has to plan around that time period.
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The language you cited simply means that in alternate years each parent gets first choice of the weeks that they want for summer vacation. If a party fails to pick there weeks on time they do not loose their right to summer visitation; they simply loose their right of priority in choosing their weeks.