If you think about it, it's obvious that magazine editors will need stories in advance of the season in which they're set. Christmas stories are published before the big day, not in the new year and the February issue of the monthly specials are often out before twelth night.
It takes time for the editor to even get around to reading our submissions and making a decision, then she must decide in which issue it will appear. Illustrations have to be found and the magazine laid out, printed and distributed.
How early then should we send our stories? Sometimes editors will provide information regarding the time scale to which they're working, or you can make a guess based on the stories they're currently buying. In the absence of any clues, I suggest aiming for four to six months ahead of when you'd hope the story would be published.
How do you handle the issue of seasonality? Do you write Valentine's tales at Hallowe'en and work on summer ones in the depths of winter? Perhaps you write them at the time they're set and hold onto them until it's time to submit them? Maybe you avoid the issue altogether by producing stories which work year round?
My own method is to write whenever an idea occurs to me, regardless of whether it's the right time, and then ... forget all about it when it comes round to the correct time of year to submit it!