There are several ways to do that:
With a rolling definition, in which you take a monthly definition (1st day of month) as a base, and than adjust by +0 using another weekly definition, which implements every sunday.
With a group definition, which implements a monthly definition (first week of month) and than add the weekly definitio of all sundays (liek above) add it to the AND section of the group definition.
But seriously, these conditions are vrey common, so we provide a global calender with definitions of these kind to all of our users for implementing them into their workflows.