Dynamically Grouping Months Based on Todays Month

Not sure how far back you want to go with your grouping, but try the below code:

Note the List argument in the List.Accumulate function -- change the upper bound depending on how many months back you wish to go

let

//create a two year table for testing
    Source = Table.FromColumns({List.Dates(#date(2021,1,1),730,#duration(1,0,0,0))}, type table [Date=date]),
    #"Added Custom" = Table.AddColumn(Source, "Quarter", each 
        let 
            pos = List.PositionOf(

//change list argument in List.Accumulate depending on how many months back you want this to go
                List.Accumulate({0..24},{}, (state, current)=> state & {
                    Date.IsInPreviousNMonths([Date], current)}),true),
            qtr = if pos = -1 then "Month 0" else
                        "Month" &  Text.From(Number.IntegerDivide(pos-1,3) * 3 + 1) & "-" & 
                            Text.From(Number.IntegerDivide(pos-1,3) * 3 + 3)
        in 
            qtr)
in
    #"Added Custom"

In this screenshot, I have filtered the results to just show the first of each month, but the original table has a row for every date.