The above diagram is a simplified representation of the average singleton's year*. Personal calamities aside, those bereft of love can trundle along on a fairly even keel for most of the year. However, there are two periods when even the most ardent of loners feel a pang of self pity.
The first of these unfortunate annual periods is centred around midnight on New Year's Eve. This really gets the new year off to a crappy start for those with no one to pucker up to in the first few minutes of it. In fact, the slump starts for most people in the run up to Christmas when there is no one special to celebrate the festive season with.
You've hardly got over that thorny period when you're plunged back into singles depression a few short weeks later. The largely manufactured Valentine's Day, corny and over hyped as it may be, still has many singletons forlornly watching the letter box, desperately hoping that someone somewhere actually fancies them. The crushing disappointment from a lack of cards, flowers, romantic candlelit dinners etc. can really grate until you become sufficiently cynical sometime in your thirties.
The good news is that from February 15th on, it's a mercifully relatively clear run for ten months until the SAD kicks in again the following December!
*In the Western world. Many around the world follow the Chinese New Year and/or do not celebrate Christmas.