Scheduling rehearsals and concerts
You should determine your rehearsal and concert schedule before you start rehearsing. You can plan an entire season in advance, or just plan one concert and its associated rehearsals; either way, you need to do this well in advance so you can get a real commitment from your musicians. You can't ask them to commit or expect them to show up unless you give them fair warning.
The number of concerts you wish to perform per season and the number of rehearsals needed to prepare for each concert depends on your musical goals and the level of musicianship you expect from your musicians. It might also be constrained by the availability of your rehearsal and performance venues. Your conductor should be heavily involved in making decisions about the number of rehearsals and concerts you will have. More than three or four concerts per year is a stretch for amateur musicians who need a large number of rehearsals and plenty of practice time to prepare the music. Eight to ten rehearsals for each concert seems to work well for an amateur-level group. Fewer than that might not give you enough time to make challenging music performance worthy. However, if you have too many rehearsals, your musicians may get bored of the music and have less fun or be less inspired to practice. Rehearsing more than once per week might make it difficult to recruit and maintain musicians; everyone has other commitments.
Of course, before you can determine your schedule, you must have first found your rehearsal and concert space. You have to work around the availability of these venues. You should also have secured a conductor, whose schedule (as well as your own) you must consider.
When scheduling a concert, make sure it won't conflict with any other local musical events your musicians might be involved in (like an existing community band's concert) with or other large events with which you would have to compete for audience (like the local professional symphony's concert, or the Super Bowl). Work around holidays, and beware of daylight saving time change days. Similarly for your regular rehearsals; eliminate any times that conflict with rehearsals of other groups your musicians may play in.
When you have eliminated as many known conflicts as you can, find out when your potential musicians are available in order to choose the most convenient time. For the RCO, I created a poll through Google Drive and distributed it to the musicians who had expressed interest in being part of the orchestra. The poll presented several time slots and days of the week and asked people to indicate their availability for each of those times. We picked the time when the most people were available. Although we inevitably had to disappoint a few people with schedule conflicts, at least we minimized overall scheduling issues and did it in a way everyone thought was fair and organized.
When you have determined your rehearsal time slot and officially scheduled your concert date(s) (and officially reserved them with the venues), you can announce the schedule to your musicians. Then, your personnel manager can start collecting official commitments from musicians. Rather than just a vague "I'm interested", you now need to get them to make an actual commitment to be at the rehearsals and the concert.
Once you have picked your rehearsal time slot, avoid changing it if possible. Your musicians need to be able to plan their schedules, and if you switch rehearsal times or days after your group is already established, you may lose a lot of musicians due to scheduling conflicts.