Sorry to be late on this.
Seasonality is less important now than it used to be. Heretofore, I would mostly avoid summer submissions and also anything in December – folk were away / na in summer, as
@Peyton Stafford mentioned above, and as far as the Xmas / Hanukkah season is concerned, well…
party!
This makes us all seem like a frivolous bunch of lightweights, but in practical terms, it often meant that other people in the decision-making chain simply weren’t available in the office. This could kill any substantial deal. So generally, I used to hold off.
Times have changed quite a bit, and working from home plus 24/7 availability via the ubiquitous cellphone,
and plus the creeping fear throughout the industry that if you weren’t constantly available you might be replaced… all these factors have made us less seasonal and more responsive to whatever crosses the transom at whatever time of day it comes in.
But still, not a bad idea to check that the agent / publisher you’re interested in is actually around. A quick phone call or email ought to do the trick, and you might learn a bit more intelligence, as well.