Summer vacation schedules can often lead to conflict between parents. Some parents, if already divorced or separated, may have a settlement agreement that provides for which parent has the right to pick first in odd/even years and a date by which they must make their decision otherwise priority in selection goes to the parent that after that date picks first. So if the agreement states that you have priority until May 15th then make your selection in writing and notify the other parent by email, this is especially important if you only have two weeks vacation time and are planning a trip. If you alternate weeks or months in the summer this is less of an issue.
If you do not have a summer schedule in place yet, then the earlier you can request your selection the better the chance of receiving consent. Likewise, if you are limited by your job as to which two weeks you have off in the summer the earlier you can notify the other parent the better off you will be. By notifying early if there is a disagreement the option of seeking a judge's decision remains a possibility. Parents that want to go to court two weeks before the planned trip may not get a court date in time or a decision in time due to the congestion on the court calendars. The goal is to avoid booking airfare without knowing if the child will be able to go on the trip.
If the trip involves foreign travel the child will need a passport which should be requested months in advance. If neither parent has an order of "sole custody" then both parents will need to sign a written and notarized consent to the foreign travel otherwise it is likely that the child will not be permitted to leave the country. The dual parental written consent or court order is often needed until age 17. There may be some variation in the age dependent upon the country of destination and departure, therefore this should be checked before the trip. The problem will usually arise at the airport with the ticket/gate agent. By that time it is much too late to do anything and all of the begging and pleading will not change the airline's decision. Don't forget to make sure that the child's passport is also provided to you at least one week before the trip if traveling abroad. If the passport is not provided in advance an emergency court application will be necessary and is subject to the schedule of the court.
If there is a special occasion taking place during your selection such as the wedding of a family member or a milestone birthday of a family member each parent should try to accommodate the other parent's desire to have the children attend the special event. On the other hand minor events should not be the cause for turning down a request especially if there are limitations in time selection caused by a parent having low priority at work for picking time off.
As is said in life what goes around often comes around. So if one parent is difficult the other parent will be waiting for the opportunity to "payback" the lack of flexibility. Parents that want what is best for their children and communicate will have a much easier time and have less stress throughout the period of raising their children. The goal is to try to do what will be best for the children and establish healthy communication between parents.