Homeowner associations declare certain weekends for neighborhood garage sales which usually attract more buyers. Finding the times can actually be easy if your HOA has a web site. More times than not, there will be a button "current rules" on their home page.
If not, simply send an email in their contact us section asking for the allowed times and any other information that you "should know".
At times the HOA will not have a policy for garage sales; this is your opportunity to get to know your neighbors. Visit with your neighbors and start organizing to get this amended to your rules at your next meeting.