Sun (actually lack of it) dictates when asters bloom. They're obligate short-day plants, meaning they need several weeks of short days to trigger flowering. They're native New Englanders that are in full bloom now.
(BTW, I'm an obligate long-day gardener, meaning that I'd rather sleep all day in the winter. Like the aster, I am a native New Englander, at least as far back as I can remember.)