Hello!
I'll try to answer this question but you really didn't provide enough background info to be real accurate.
You mentioned your business is seasonal, because of this factor many seasonal businesses cannot control the pricing not the demand as much as their competitor could. For your fyi: this called a perfect competition.
Anyway, I think you would be best off promoting right before the season begins. You don't want to start advertising your product or services right when others are already making the purchasing decisions... You want to drive to first buyers to your business in hopes that they provide reviews and referrals to their friends so then they come to you.

The timing would be maybe a month or few weeks before the season & overlapping into the season by a month or month & 1/2... Depending on what your business is and how long the "season" lasts. If is almost through the entire year then yeah keep promoting for longer.

Another word of advice: The efficiency of a marketing campaign depends on what's used for that market.. Tv, email, blog, referrals, surveys, landing page, newspaper, fb, etc... Consider this before you spend your money in the marketing itself :)