AFAIK, in talking to people who have already gotten their first dose (lucky them - I still can't get an appointment) you have an appointment for your second dose before you leave the facility - at the same time, ie, if you got your first dose at 2:15PM you will get your second one 21 days later at 2:15PM.
Two first come, first served facilities, opened 8 hrs/day 6 days a week-or until 500 doses are given- are opening Monday, but are based on your birth month - Monday for Jan/Feb, Tuesday for Mar/Apr, and son on. Photo ID is required with DOB is required. It's for anyone over 65, or any healthcare workers of any age who must also present their work ID. This is in Duval County/Jacksonville.