Answer: If the shrubs are container grown, they can be planted any time during the growing season or when the ground is not frozen. Bare root plants are usually only available in early spring and that is the best time to plant them. Snow helps insulate the plants and is not a problem, with the possible exception that it could slide off the roof and smash the plants.
Many varieties of lilac and forsythia are very large shrubs (over eight feet tall and wide at maturity), so think more about the scale of plants in relation to your house and yard. There are smaller varieties of both of these shrubs that may work better for you. Make sure you select a forsythia variety that has flower buds cold hardy in your area. The shrubs themselves are very hardy, but flower buds are susceptible to winter kill.
Here's a list of smaller alternatives which are very amenable to being shaped and trimmed to make neat foundation displays: the shorter types of juniper, red and gold barberries, the smaller littleleaf lilacs, and the smaller summer-blooming spireas, with blooms in shades of pink.
Q&A Library Searching Tips