When a court determines the visitation rights of a noncustodial parent, it usually orders "reasonable" visitation, leaving it to the parents to work out a precise schedule of time and place. This allows the parents to exercise flexibility by taking into consideration both the parents' and the children's schedules. Practically speaking, however, the parent with physical custody has more control over the dates, times, and duration of visits. That parent isn't legally obligated to agree to any particular schedule, but judges do take note of who is and who is not flexible. If you are uncooperative merely to vex your ex, it can backfire when you need to ask the court for something in the future.
Parents have to cooperate and communicate frequently for the reasonable visitation approach to succeed. If you suspect that a loosely defined reasonable visitation won't work, insist on a fixed schedule and save yourself time, angst, and, possibly, money. If you've already agreed to reasonable visitation and it isn't working out -- for example, one parent is consistently late, skips scheduled visits, or doesn't inform the other parent where he or she is planning on taking the children -- you can go back to court and ask that the arrangement be changed.