Selecting latest file from current and past months

Hi Anonymous 

You can groupby to find the max date for each month

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjDXNzDRNzIwMlSK1YlWMjRE4RoZo3ANzPQNTJG4FihcIwNUWaBiMySTDVC5JghuLAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", type date}}),
    #"Inserted Month" = Table.AddColumn(#"Changed Type", "Month", each Date.Month([Column1]), Int64.Type),
    #"Grouped Rows" = Table.Group(#"Inserted Month", {"Month"}, {{"latest", each List.Max([Column1]), type nullable date}}),
    #"Merged Queries" = Table.NestedJoin(#"Inserted Month", {"Month"}, #"Grouped Rows", {"Month"}, "Grouped Rows", JoinKind.LeftOuter),
    Custom1 = Table.SelectRows( #"Merged Queries", each [Column1]=[Grouped Rows][latest]{0})
in
    Custom1

 or still use your way, 

#"SelectLastDateofMonth" = Table.SelectRows(#"Changed Type8", (sel)=> sel[Index]=List.Max(List.Select(#"Changed Type8"[Index], each Date.Month(_)= Date.Month(sel[Index]) and Date.Year(_)= Date.Year(sel[Index]))))