You definitely know you can throw your garage sale at the time your community is throwing a general one if you live where this is being done, as you will have access to many shoppers. You can also check your Homeowner association for any garage sale tips as they might have some.
Each location has a different date for garage sales, some areas has their own every Friday and Saturday from 8 am. Most areas love to do this in the morning. Shoppers tend to reduce towards noon, so it is important you consider ending your garage sale at noon if you can.
Before you pick a date for your garage sale, consider having it in two days rather than just one so you can make more sales. More available shopping time equals more shoppers at your garage sale.