Because they work and are cost-effective. At a response rate of one-half percent, a mailing to 10,000 would bring you 50 registrants. But the real measure of profitability is how much income, in terms of registration fees, the mailing generates in excess of your direct mail costs. My own experience is that the best seminar seasons are March through May and September through mid-November.
Summer interferes with vacations and winter brings the danger of cancellation due to bad weather.
Based on a thorough study of the seminar mailings that cross my desk, I would say that most arrive in my mailbox four to eight weeks in advance of the date. I have frequently had seminar attendees tell me they signed up for my seminar because of a single bullet item that appeared in this list. Beyond that, your best bet is to save all the seminar mailings you receive and study the formats of the ones that you receive over and over. These are the proven winners that have developed a copy and graphic approach that works, and you would do well to learn from their success. Most seminar leaders recommend this, but I hold a contrary opinion.