Temperature and day length trigger synchronous re-emergence in spring, in time for the arrival of food. Ladybirds have to wait for unfurling leaves, and then for the aphids to get going too.
In Northern Europe temperatures above 12°C rouse the common 7-spot, but in North America the convergent ladybird needs over 15.5°C. Emergence is not precise, however. This is a hedge against freak weather awakening a group before more typical winter conditions return.