While Marvin supports a wide variety of recurring task patterns (like daily, weekly, and every X days), and even custom recurrence patterns like "every 3 weeks on Tuesday", there are some recurrence patterns that are hard to express with words. Marvin allows arbitrary RRULE recurrence patterns (RRULE is an industry standard solution for describing recurring event patterns).
First, open the recurrence pattern generator here: https://amazingmarvin.github.io/rrule/
You can then use the builder on the left to create complex recurring patterns. For example, if you want to recur on the first day of the month (excluding Fridays, Saturdays, and Sundays), it would look like this:
You can hover over the fields to learn more about their meanings.
On the right you will see upcoming dates:
As well as the "Pattern" that you can paste into Marvin:
Be sure to copy and paste the entire text, including "RRULE:" at the beginning.