You'd use the formula during the load of your calendar in the script, or wherever you're loading your Month and Year fields. I'm assuming that your calendar has a field Date. In any case, you'd use the same Date field that you're using to establish the Month and Year, assuming you're doing it that way. So you might have something like this:
[Calendar]:
LOAD
Date
,year(Date) as Year
,month(Date) as Month
,date(monthstart(Date),'MMM YYYY') as "Month Year"
;
LOAD DISTINCT Date
RESIDENT [My Data]
;
There are more efficient ways to get the list of distinct dates, but they're also more complicated, so I figure this makes for a better example. I'll also usually generate my calendar for every day in the range, not just for dates for which I have data, but that's more complicated still, so again, trying to keep the example simple.
.png "Luminary Alumni"){kind=icon}