You can do a search for something like "astronomy" or "astro*" and then sort by date on TheGradCafe's Results database. Here's an example search: http://thegradcafe.com/survey/index.php?q=astronomy
That would give you a good overview of astronomy results timings. Another good way to use it is to search for each of the schools you are applying to, e.g. "Berkeley Astronomy" and then you'll just see user-submitted results for that school. Schools will generally give results around the same time each year.
I still can answer your question generally though. Many schools will give results between mid/late January through early March. The higher ranked the school, the earlier they will tend to release results. Lower ranked schools may wait until the top ranked schools provide results first, since people who would get into both would likely choose the top ranked school so this way, they know to give the spot to someone else.
This year, the AAS meeting is quite early and many of the top schools will be using this conference to meet candidates. So many committees etc. won't meet until early January (after the conference) to debate candidates.