A calendar month is just that: a month from the calendar which will have a name such as May or June.
28 days is a lunar month and is not relevant (unless you're an astronomer).
So you count back to the beginning of the month six months before the month preceding your flight.
If you were flying tomorrow therefore you would look for six approaches from the 1st April.
And of course you would have done your holds, VOR /NDB interception and tracking as well.