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Hi rima_ch ,
Go to power query and paste this into the advanced editor:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjTSN9Q3MjC0VNIBsY1BHCMTpdhYAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Start = _t, End = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Start", type date}, {"End", type date}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Dates", each {Number.From([Start])..Number.From([End])}),
    #"Expanded Dates" = Table.ExpandListColumn(#"Added Custom", "Dates"),
    #"Changed Type1" = Table.TransformColumnTypes(#"Expanded Dates",{{"Dates", type date}}),
    #"Added Custom1" = Table.AddColumn(#"Changed Type1", "Year", each Date.Year([Dates])),
    #"Added Custom2" = Table.AddColumn(#"Added Custom1", "Month", each Date.MonthName([Dates])),
    #"Changed Type2" = Table.TransformColumnTypes(#"Added Custom2",{{"Year", type text}, {"Month", type text}}),
    #"Removed Other Columns" = Table.SelectColumns(#"Changed Type2",{"Year", "Month"}),
    #"Grouped Rows" = Table.Group(#"Removed Other Columns", {"Year", "Month"}, {{"det", each _, type table [Year=nullable text, Month=nullable text]}}),
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"det"})
in
    #"Removed Columns"

Let me know if you have any questions.

If this solves your issues, please mark it as the solution, so that others can find it easily. Kudos 👍are nice too.
Nathaniel