Counting how many of a specific weekday in a specific month

Other than use brute force method to loop through the month to count days, it can be calculated as below (in Java Script):

Math.floor((dayInMonth - ((w - weekdayOfDay1 + 7) % 7) + 6) / 7)

Where dayInMonth is the number of days in that month, w is the day of week (e.g. Sunday) we want to count and the weekdayOfDay1 is the day of week on the first day of that month.  Notes, all "day of week" are mapped as 1 for Monday, 2 for Tuesday…, 7 for Sunday.

As the set of results for is limited, pre-calculated values can be used and simple arrays or mappings can be loaded in programs to get the result directly as an alternative method.  

The mapping table is pretty small (196 entries):

7 (possible day of week to be counted) X 7 (possible day of week on the first day of the month) X 4 (possible number of days in a month) .

e.g. table for first day of the month is Monday:

wday\nday	28	29	30	31
1	4	5	5	5
2	4	4	5	5
3	4	4	4	5
4	4	4	4	4
5	4	4	4	4
6	4	4	4	4
7	4	4	4	4

Not sure about the performance differences.

BL