The basic step is to unpivot all except the first two columns. Then extract the month number and combine it with the Year to define the date.
Here's a full sample query you can paste into the Advanced Editor of a new blank query to examine the steps:
let
Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjIwMlTSUfI1NACSlgaqQNIUiYSIGBrg4mCShkAyVgfJYBCJYS6ySRhsNALJRCOwiUZUN9GYKibGAgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Year = _t, Group = _t, M_Checklist_01 = _t, M_Checklist_02 = _t, M_Checklist_03 = _t, M_Checklist_04 = _t, M_Checklist_05 = _t, M_Checklist_06 = _t, M_Checklist_07 = _t, M_Checklist_08 = _t, M_Checklist_09 = _t, M_Checklist_10 = _t, M_Checklist_11 = _t, M_Checklist_12 = _t]),
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Year", Int64.Type}}),
#"Unpivoted Other Columns" = Table.UnpivotOtherColumns(#"Changed Type", {"Year", "Group"}, "Month", "M_Checklist"),
#"Extracted Last Characters" = Table.TransformColumns(#"Unpivoted Other Columns", {{"Month", each Number.From(Text.End(_, 2)), Int64.Type}}),
#"Added Custom" = Table.AddColumn(#"Extracted Last Characters", "DATE", each #date([Year], [Month], 1), type date)
in
#"Added Custom"
