Average number of work days per month including vacation days?

21.67 days per month. There are 52 weeks in the year with 5 working days each = 260 days, and when divided by 12 months = 21.67 days. Weekday holidays are also included.

I would say this to be more a accurate calculation:

Would it not be correct to say on average there are 365.25 days a year (365 normally 366 every 4th year). With 7 days in a week there would be 52 weeks and 1.25 days. That extra 1.25 days is why the day of the week slips one calender day ahead every year, except on leap years which slips two days ahead after the leap day. 52 weeks would be 260 weekdays (52 x 5) but then we are left with that pesky 1.25 days which is not always a weekday or weekend. 5 out of 7 days are weekdays so you would multiple 1.25 by 5 and divide that answer by 7 (1.25 x 5 / 7 ≈ 0.9). Adding this to the already determined 260 days will give us the average number of weekdays a year. So the average number of weekdays a year would be approximately 260.9. Over 28 years this number works out as whole number (7305 weekdays) which makes sense since it would take 7 leap years (which happen every 4 years) to complete a cycle of a leap years starting on different days of the week.

With just over 52 weeks a year the calender should have had 13 four week months, but since 13 was such an "evil' number 12 months was chosen instead. They created a entire hodge-podge of month lengths; 7 months of 31 days (4 weeks 3 days), 4 months of 30 days (4 weeks 2 days) and February with 28 days (exactly 4 weeks) except every 4 years when it is a day longer. Dividing 260.9 days by 12 gives us about 21 and 3/4 weekdays a month, but that will only be truly accurate over 28 year periods. If you use that average to say how many weekdays there are in February you'd be off by almost two days since there will always 20 weekdays in Feb. In month with 31 days depending on the day of the week the month starts you could have as little as 21 days and as many as 23.

Weekday holidays vary from place to place and job to job so there is no way to calculate that into that average and have be valid for all places in the world. You would have to take the number of weekday holidays that you have in a year and subtract that from 260.9 and then divide that answer by 12.

As an example lets say you have 2 weeks vacation a year and 7 bank holidays. That would be 17 total days off a year. (260.9-17)/12 is approximately 20 5/16 days. Which, as I said before, is an average and only truly accurate over a period of 28 years and therefore could be very different from the actual number. As an example consider this; if you take your two week vacation In February and that month has a bank holiday as well you've actually only worked 9 days that month not 20.325 days.