Is it just me that finds that Christmas doesn't start in the shops quite so early as it once did? (like September or October!), did the public backlash at being told Christmas is only two months away finally kick in? or does the speed at which things can be done online these days make it pointless to prepare for a special event like Christmas more than a month in advance?
Traditionally designers often started their Christmas jobs in July-Sept, hence traditional stock sales of Christmas images started early, and by December design offices were quiet places with few new jobs appearing till the New Year. Creation of seasonal stock images was a year round event, and submissions might be at least 6 months in advance. The same was true for a lot of other special events during the year which require images, St. Valentines Day or Halloween etc.
Microstock sales behave in a quite different way.
I run a couple of seasonal (Christmas and Halloween) image sites, and notice that on both of these sites activity increases right up to the event (even including the day of the event!) then sharply drops off afterwards. Why? Because microstock images can be used by a different type of designer working in a different time-frame. Designs these days can be launched online or via email in hours rather than needing weeks for printing and proofing. It's quite possible these days to make Christmas image sales a few days before Christmas to someone needing images for a last-minute campaign.
It seems that many microstock buyers have a more 'consumer orientated' buying approach, e.g. only thinking about Easter a few weeks before it occurs, and certainly not before valentines is out of the way (even while valentines might just be a month before). Many buyers, especially those e-marketeers work with very short deadlines only days before the event may the need for extra material become clear. A lot of online publishers only think about grabbing an image just before new content is created, while that might not be the best or most organised approach, as a microstock photographer we only need to cater for their need for 'something to throw in there'. (Where did the pride in my work just go to?!?)
I'm not suggesting that designers don't plan or buy microstock images well in advance of an event, but with the way that a lot of agencies skew their search results for more recent submissions then submitting seasonal images six months in advance might not be the best option for microstock. Of course if you submitted seasonal images last year they will be available for those early birds, but with the bulk of sales appearing closer to the event then submitting about 8-14 weeks before a big event should ensure maximum visibility.
The Big Events Calendar
(in rough order of sales volumes - open to comment on this!)
Christmas is the second most searched for microstock keyword (ref ), Competition is very high for Christmas images, and the high quality of the images already available might make Christmas not the best choice to start out with. Get those festive images in well before the end of October.
It moves around between late March and April, and lots of people skip the religion and go for the chocolate eggs. There is a lot of contention about keywording this one as so much of the non-religious Easter image subjects are generalised (giving the keyword 'Easter' to lambs, spring chicks or crocuses). Submit images in January or February.
Valentines The marketers money making invention. This one creeps up on you; get those images submitted first thing in January if not the December before.
4th July / National Patriotic Imagery, these can often be submitted all year round (fireworks, flags and national monuments) Submit in April or May, depending on public holiday in your country.
Halloween, Mostly an event in the United States but known all over the world (despite not being traditional) mostly as good excuse for a dress up party and some extra microstock sales for all that marketing material. Submit in September.
Thanksgiving, While many countries celebrate some kind of harvest celebration or festival, it seems only the US and Canada turn the event into a serious public holiday, though there are other national variations - sans the pilgrim fathers. Submit images in August.
Mothers and Fathers Days These cause a bit of a problem as they fall at different times in different countries that celebrate them. They are also typically satisfied by more generic family images alongside the cheesy silk hearts with 'mum' written on them. February (perhaps ?).
New Years Eve, call me cynical, but apart from the next year lit in lights, then most picture requirements for new year are catered for with images depicting 'celebration' 'renewal' 'birth' and 'party' or yet more fireworks photos... Upload October/Nov.
Hanukkah, Diwali, Eid.... and all the others. There is a whole world of celebrations to create images for, it's often easier to create images for national days in your country or belief system than to try to take a photo suitable for something you understand little about. Example: collecting props for an American style Thanksgiving image if you live and work in Brazil. You also have a better understanding of the meanings of each event if you are involved in them. Sales volume for these local events is smaller, but there is also less competition on them so sales revenue can be similar.
The Long-Tail Sales
Not long ago I used to feel that you could submit microstock images designed for seasonal events year round, and while for quite a few of the agencies that true; for others especially shutterstock, submitting before the event will generate increased sales on searches of those keywords. Of course if your images are good then they should continue to earn sales at the same time each year for several years to come. Adding images before an event simply means that they will receive more views, and depending on the site also be the images that buyers see first when they view your profile.
Help from Google Trends
Google trends  is a free Google service that plots changes in the number of people who enter search terms into Google. It won't tell you how many people search for a term but you can compare terms to each other. It's also difficult to think of a search terms that have enough data and are also tightly scoped enough so they only include stock photography related searches, remember these are results from Google as a whole not just from people looking for stock photos. The Google trends plots do make some interesting viewing provided you understand the limitations in their accuracy.
Click on one of the examples below to visit Google trends and try your own terms (Graphs captured May 2009)
Even while the Christmas search peak is in early - mid December, the bulk of the search volume (area under the graph) is back in November and October.
For me these graphs pose more questions than they answer; are the few people who search well before an event the professional 'buyers with money' and the masses who search right up to the event date just consumers looking for something free at the last minute?
Having a good Christmas selection in your portfolio can offset poorer sales of other images during December.
You might think about taking photos one year (or consider buying post event discount items for photo props) and then submitting them the next. It's much easier and cheaper to get Christmas decorations in the January Sales than find them in August. Taking images at the event while people are in the 'spirit' can prove easier than getting amateur models to act out at a different time of year.
Despite what you might expect all is not lost if you only get your image in a few weeks before the event, (but I recommend longer). With some sites taking 14 days to review images then you have pretty much missed the Christmas boat after the first week of December.
Don't Submit seasonal event specific images a couple of months after the event, it looks amateurish to have 'new images' in your portfolio that are months out-of-date / ten months too early.