End date based on next row (Index+1) for each row

Hi again mthiru, I've added 1 more row to sample data and updated a code (in prev. post). Let me know it this is what you need.

It is correct from my point of view, because you've asked for: 
"if a transaction occurs for the same month, then next row start date minus 1. The dates have to be a business day"

In the first row you have date 31.12.2022 which is in the same month as second row (again 31.12.2022) and you wanted to return last business date before 2nd row date which is 30.12.2022 - Friday.

You're welcome. Here is updated code - matching with new sample:

let
    fnShift = (tbl as table, col as text, shift as nullable number, optional newColName as text, optional _type as type) as table =>
        //v 3. parametri zadaj zaporne cislo ak chces posunut riadky hore, kladne ak dole, 4. je nepovinny (novy nazov stlpca), 5. je nepovinny typ
        let
            a = Table.Column(tbl, col),
            b = if shift = 0 or shift = null then a else if shift > 0
                then List.Repeat({null}, shift) & List.RemoveLastN(a, shift)
                else List.RemoveFirstN(a, shift * -1) & List.Repeat({null}, shift * -1),    
            c = Table.FromColumns(Table.ToColumns(tbl) & {b}, Table.ColumnNames(tbl) &
                ( if newColName <> null then {newColName} else
                    if shift = 0 then {col & "_Duplicate"} else
                    if shift > 0 then {col & "_PrevtValue"} 
                    else              {col & "_NextValue"} )),
            d = Table.TransformColumnTypes(c, {List.Last(Table.ColumnNames(c)), if _type <> null then _type else type any})
        in
            d,

    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("tdI9C4MwEAbgvxIyG7i7fJm9dBA6dRQHLYEulRIU6r+vGopQYsFA3yEhB3lyHKlr3o+Pzgde8MG/hnk7+ZtfKkxiwQiI5hoiCAB0QqAV8bxdQLcsdisIkgoclGh4U/zjgYM2fdn0yy7JOW0UJvmq7cc2TIz0qsvDOsKaJH72XYi6zdQVaTknqV/acLszUrmNmyV2h54+8z4Oa+1wr+dq7D2TkCnLmKR89c8hfpQyV7cG14E0bw==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [EC = _t, Name = _t, AccountingDate = _t, Ledger = _t, MainAccountId = _t, AcctDescription = _t, BU = _t, PV = _t, Description = _t, Amount = _t]),
    ChangedType = Table.TransformColumnTypes(Source,{{"AccountingDate", type date}}),
    Ad_PrevWorkingDate = Table.AddColumn(ChangedType, "Prev. Working Date", each List.Max(List.Select(List.Dates(Date.AddDays([AccountingDate], -1), 3, #duration(-1,0,0,0)), (x)=> Date.DayOfWeek(x, Day.Monday) < 5)), type date),
    GroupedRows = Table.Group(Ad_PrevWorkingDate, {"MainAccountId", "BU", "PV"}, {{"All", each 
        [ a = fnShift(_, "Prev. Working Date", -1 , "PWD Shift"), //Prev. Working Date Shift
          b = fnShift(a, "AccountingDate", -1 , "SD Shift"), //Prev. Start Date Shift
          c = Table.AddColumn(b, "EndDate", (x)=> if Date.ToText(x[AccountingDate], [Format="yyyy-MM"]) = Date.ToText(x[SD Shift], [Format="yyyy-MM"]) then x[PWD Shift] else Date.EndOfMonth(x[AccountingDate]), type date), //Add EndDate
          d = Table.RemoveColumns(c, {"Prev. Working Date", "PWD Shift", "SD Shift"}) //Remove helper columns
        ][d], type table}}),
    CombinedAll = Table.Combine(GroupedRows[All])
in
    CombinedAll

If you want to understand the logic of previous/next row value, read this article.