The standard guideline is that the weekend's place in the month for custody purposes is determined by the date on Friday since that's the day when weekend possession begins. So for example, if the first day of a month falls on Saturday, that is not the first weekend of that month.
The months with 5th weekends are going to change from year to year depending on how the months happen to fall with respect to the days of the week. For instance, this year February has a 5th weekend due to it being a leap year with leap day occurring on Friday. This weekend is the 5th weekend of February (as opposed to the 1st weekend of March). Next weekend will be the 1st weekend of March.
Hope that doesn't thoroughly confuse you, but using the Fridays hopefully clarifies it a little bit.