See the attached. It uses a nested table calculation with an Advanced Compute Using of Week of Order Date. The inner calculation (Avg Rest of Month Sales) generates a sum of the sales for the 2nd through Nth (4th, 5th, or 6th) weeks of the month, then divides that by the # of weeks-1 to get the average of sales. That is wrapped in PREVIOUS_VALUE so that it is only calculated once per partition (per combination of Month, Year, and any other dimensions that are part of the partitioning). The outer Sales Comparison measure is based on an IF statement that checks if the week is the 1st week, if so it returns the sum of sales, otherwise it returns the Avg Rest of Month Sales.
There are a lot of variants that can be created of this, I tossed together a couple as well.