To calculate the fourth sunday we must first calculate the first Sunday in current month. Cell B3 contains this date 1/5/2019.
WEEKDAY(B3, 2) returns a number representing the weekday. 1 is Monday, 2 is Tuesday and so on.. 7 is Sunday.
7 minus 6 equals 1. We must add 1 to the date in cell B3 to get the first Sunday in that month. (This calculation works only if the date in cell B3 is less or equal to the date of the first Sunday in that month.)
To get the fourth Sunday we add 21 to the date of the first Sunday.
1/5/2019+1+21 equals 1/27/2019.
Step 2 - Calculate 4-th sunday next month
To get the first day of the next month we need to calculate the first date of this month and then add 1 to the month.
DATE(YEAR(B3), MONTH(B3)+1, 1)
DATE(2019, 1+1, 1)
DATE(2019, 2, 1)
and returns 2/1/2019. This date is also used to calculate the fourth Sunday.
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Article with the 'SUMIFS' Function
The following article has a formula that contains the SUMIFS function.