Best time to plant a garden contained by Upstate, NY?

Last year, I planted in May and still encounter frost.
My lilac bush never blooms?
Answers:    My mom always wait until Memorial Day Weekend and never had to verbs much !
How speedily do "Wave Petunias" spread?
The rule of thumb for this area is Memorial Day and after. If threat of frost exists, cover seedlings/plants at hours of darkness and uncover for the afternoon.