Garlic should go in the ground in fall, October to mid-November. It's a little late now, although I do have some sprouting in the kitchen that I will probably jam into the dirt and hope for the best. They probably will grow and form bulbs, but won't be as vigorous and will make smaller bulbs than if you had planted them last fall.

Onions on the other hand... depends on your specific climate, and whether you're planting seed or sets. The trick with onions is to avoid letting them go through a prolonged cold spell, which triggers bolting (after it warms back up).