Custody in General
An odd-even schedule comes into play after an overall custody arrangement is established as part of your divorce. You can do this by agreement with your spouse, or the court can order custody according to the best interests of your children -- a series of factors meant to determine in whose custody your children are most likely to thrive. Whether you share custody, or one of you has physical custody and the other parent has visitation, you must still decide how you're going to handle holidays and special occasions. Courts often default to an odd-even schedule for this, which attempts to arrange the schedule in such a way that neither parent is faced with never spending holidays with their kids.
Determination of Odd and Even
The last number in the calendar year determines whether it has an odd or even designation. For example, 2013 is an odd year, while 2014 is an even year. The year's holidays are then categorized into Group A or Group B. You might have Group A in even-numbered years, and Group B in odd-numbered years, with your spouse taking Group A in odd-numbered years and Group B in even-numbered years. This ensures that neither you nor your spouse will have the kids all the time, for every important holiday all year. Courts in some states may default to this type of system when parents can't reach a parenting plan by consent as part of their divorce, but you can design the same arrangement if you're negotiating custody as part of a settlement agreement.
Allocation of holidays between Groups A and B should aim for fairness. For example, you wouldn't want to put Christmas or Hanukkah together in the same group with Easter or Passover. Courts typically don't do this because if these holidays are in the same group, one parent must go a whole year without sharing either one with the kids. Parents and courts often pair Thanksgiving with the spring holiday so neither parent has to be without the kids for the entire stretch of winter occasions. You can get particularly creative with Christmas; you might have Christmas Eve in odd years, and Christmas Day in even years, with your spouse taking the opposite time.
Other Special Days
Look at your calendar and identify all the three-day weekends that fall over the course of the year, such as Labor Day, Memorial Day and President's Day. The fairest way to account for these is to split them up over two groups, so each parent has one or two a year. You'll also want to account for the Fourth of July and Halloween. Mother's Day and Father's Day are typically exempt from the odd-even schedule – these days should be spent with the appropriate parent. The odd-even schedule can work with your children's birthdays if the day itself goes to one parent in odd years, with the other celebrating the day before or after, then reversing in the even years. All holidays and special occasions generally override your regular visitation plan; if one of them falls on a day when you're supposed to have the kids, the holiday schedule prevails and you may lose your day. By the same token, you'll sometimes get your kids on days that may not have been your scheduled time because of the holiday schedule.
Summer Vacation and School Breaks
If you're unable to reach a parenting plan by consent with your spouse, the court generally will not apply the odd-even system to summer vacations and spring and Christmas breaks. However, this doesn't mean you can't do so in a custody agreement if it works for you. Typically, the court allots blocks of weeks to the non-custodial parent in the summer, and divides long school breaks into equal chunks of time with each parent.