First developed in the early 1960s by behavioural psychologist John S. Adams, equity theory is concerned with defining and measuring the relational satisfaction of employees.
For 31 days in a month: (31 days/month) / (7 days/week) = 4.4286 weeks/month. For 30 days in a month: (30 days/month) / (7 days/week) = 4.2857 weeks/month. For 28 days in a month: (28 days/month) / (7 days/week) = 4 weeks/month. For 29 days in a month: (29 days/month) / (7 days/week) = 4.1429 weeks/month.
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.