Difficult question.
Depends on which you're talking about.
Confusion with the new names.

There's attractive annual Callistephus (Aster) chinensis (China aster).

Earlier blooming low asters include Aster alpinus (alpine aster), which is spring blooming. Aster tongolensis (East Indies aster) is mid/late summer.

Symphyotrichium (Aster) divaricata (white wood's aster) is quite low and begins blooming in mid/late summer. Picture below is Sept 2, 2010.

The earliest taller aster (find it always needs staking) I know of is Aster x frikartii (e.g. 'Mönch'), starts blooming in late summer.

The big Symphyotrichium (Aster), S. novae-angliae (New England asters) and S. novi-belgii (New York asters) and hybrids are all fall blooming.

This post was edited by SunnyBorders on Mon, Sep 16, 13 at 17:03