Start them soon! Peppers are SLOW to germinate and need a consistently warm spot to do so, so the heating mat will help!
Soaking the seeds for a few days prior to planting helps, as does pre-sprouting in damp paper towels.
I planted mine a week ago and so far nothing - but that’s normal. I moved my lighted grow rack to a warmer part of the house, and that has given me better germination the past two years.
More ideas: https://growhotpeppers.com/germinating-pepper-seeds/
The above link is about hot peppers, but it applies to sweet peppers, too!
Thanks for the info! The date calculator on that page says I’m a month late :)
Valuable link, thanks!