Home / Settings / Delivery Schedules / Add Custom Schedule.
Name your schedule and select the "Addons Only" checkbox if this schedule is specifically for add-on items (otherwise leave this box unchecked.)
On the following page, use the calendar on the right to add weeks, this can also be edited after creation. (But it cannot be renamed, for that create a new custom schedule.) Once a custom schedule is Activated, it will display as one of the frequency options for product set-up.
Custom schedules allow you to offer more options, like every 3 weeks, or twice a month on the first and third weeks, or even the 15th of every month. Some of the restrictions are: NO proration for whole season shares, the schedule might need an explanation in the product long description, you cannot use the week forward/backward adjustment. Also, when using custom schedules, pay attention to attached add-on subscription frequencies as they are ordered and scheduled.