If you want to schedule a volunteer exactly one time per month in a specific ministry on a specific weekend, you will want to set up a Preassignment. For example, you can schedule Pat Smith as Lector 1 the first weekend of every month.
If you want a volunteer to be scheduled approximately once per month on any given weekend each month in a specific ministry, you will want to change that volunteer's preferred serving frequency. By using the "set by ministry" option, you can say you want Pat Smith to serve as Lector one time per month. It should be noted that preferred serving frequency is an approximation of how often a volunteer will be scheduled and can change due to other factors in your database.
If you need help figuring out how to best schedule a volunteer once per month, please send in your data files to tech support (Help - Send Data Files to Tech Support) and explain to us the situation.