When I used to advertise Id hit the targeted subdivisions last week of Jan, last week of Feb. and the 3rd week of March. I also had a add in our "county" paper year round.
Gernerally about the first week of April everyone calls wanting everything done before Easter. This year Im not doing any 1 time jobs once the season starts.
Nothing really scientific about it, Last year the season started 1 month early.
The trick is to try to get people to do stuff in Jan and Feb, althought its hard. If you like to do 1 time jobs keep all customers phone # and address and calll them next year early.