Christmas has long been recognized as the peak time for marriage breakdowns for various reasons including the couple's forced proximity and having to put on a happy public face. Christmas does not itself cause marriage breakdowns, in many cases it is just the final straw.
Christmas is a time where normal family routine is put on hold for a week or longer. Both partners are around the home in because of the public holidays. If a relationship is under stress and the partners have not addressed any long-standing differences before the Christmas holiday, the enforced proximity will cause stress and bring their marital problems to a head.
Marriage breakdowns happen when a husband and wife stop loving each other. Without the love, they are unwilling to make the everyday compromises that are necessary whenever two people live together. It is only the willingness of both parties to make small compromises that prevents marriage breakdown. Without compromise, every everyday situation leads to conflict.
Christmas is a time when everyone has their own expectations and in a loving marriage both partners happily make compromises and resolve any potential conflict before it develops. If the marriage has started to break down then the couple's communications with each other have usually suffered. If there is no communication then reaching a new equilibrium in the marriage becomes impossible and breakdown becomes inevitable.
Having to pretend to be happy parents for the sake of children's happiness at Christmas puts another strain on the marriage. Neither partner wants to 'spoil Christmas for the children', so both put on a brave and happy face. Unfortunately this causes extra resentment when the couple are alone, which leads to arguments and an outbreak of open hostilities that have simmered just below the surface up till now.
Children are very sensitive to atmosphere and are almost certainly aware of the tensions between their parents, so the couple are deluding themselves that their children will have a happy Christmas. If the situation has reached this point then re-establishing communication and a willingness to make compromises becomes impossible and this will almost certainly be the final Christmas before the couple recognize the reality of their marriage breakdown.
If there are no children, or if the children have left home, then the couple's resentment is still likely to peak at Christmas. If one partner normally works in the home, then he or she often resents the unusual presence of the other person, disturbing the normal home environment.
The enforced proximity forces both partners to recognize their imminent marriage breakdown. It is not pleasant to have to recognize that your marriage, on which you pinned so much hope, is in breakdown. At this stage of a marriage the partners are often living separate but parallel lives in the same house, and like parallel lines, never coming together or touching.
With husband and wife both forced to be in the same small space for the holiday season their parallel lives are forced into contact with each other and unless they address the reality of their marriage breakdown then divorce or separation become inevitable.