The CHOOSE and WEEKDAY functions allow us to craft a formula that returns the first Sunday in a month.
If you type a different month-end date in cell A1, such as 1/31/2015 then cell B3 will return 2/1/2015, as shown in Figure 4. The first Sunday of February 2015 happens to fall on the first of the month.

I hope this helps!
Tim Paine