Power Query

easy 

let
    // get date from day, month and year
    get_date = (lst) => Date.From(Text.Format("#{0} #{1} #{2}", lst)),
    // generate consequtive dates and add dates column to original table
    fx = (tbl) => [
        rows = Table.RowCount(tbl),
        year = tbl{0}[year],
        month = tbl{0}[month],
        days_in_month = Date.DaysInMonth(get_date({1, month, year})),
        dates = List.Generate(
            () => 1, 
            (x) => x <= rows,
            (x) => x + 1,
            (x) => get_date(
                {
                    ((d) => if d = 0 then days_in_month else d)(Number.Mod(x, days_in_month)),
                    month,
                    year
                }
            )
        ),
    z = Table.FromColumns(Table.ToColumns(tbl) & {dates}, Table.ColumnNames(tbl) & {"date"})
    ][z],
    // this is sample table, use yours
    Source = #table(
        type table [year = number, month = text, some_data = text],
        List.Repeat({{2025, "January", "jan data"}}, 100) & 
        List.Repeat({{2025, "February", "feb data"}}, 300)
    ),
    // group data by year and month, apply fx function
    group = Table.Group(Source, {"year", "month"}, {"x", fx}),
    // combine year-month tables
    result = Table.Combine(group[x])
in
    result

Try this:

After grouping by month and year, we use an Index column and the Number.Mod function to calculate the relevant day of the month according to your rules

let

    //Create Sample Table 100 rows Jan; 50 rows February; 50 rows March
    //You should replace this section with your own table
    ColumnHeaders = {"Year", "Month","Category","Description","Value"},
    Year = List.Repeat({2025},200),
    Month = List.Repeat({"January"},100) & List.Repeat({"February"},50) & List.Repeat({"March"},50),
    Category = List.Repeat({"Expense"}, 200),
    Description = List.Repeat({"Food"},200),
    Value = List.Repeat({147.28}, 200),
Source=Table.FromColumns({
        Year , Month , Category , Description, Value}, ColumnHeaders),

//Group by Year and Month Columns
    #"Grouped Rows" = Table.Group(Source, {"Year", "Month"}, {
        {"Date", (t)=> [a=Table.AddIndexColumn(t, "Index",0,1,Int64.Type), //Add Index Column
                        b=Table.AddColumn(a,"Date", each //add date column will = first of the month
                            Date.FromText(Text.Combine({Text.From([Year]),[Month]}," "))),

                        c=Date.Day(Date.EndOfMonth(List.First(b[Date]))), //Number of days in relevant month

                        //Transform the date column into a date using the Index column and the 
                        // Number.Mod function to calculate the relevant day
                        d=Table.ReplaceValue(
                            b,
                            each [Date],
                            each [Index],
                            (x,y,z)=>Date.AddDays(y,Number.Mod(z,c)),
                            {"Date"}
                        )][d],
                        type table [Year=number, Month=text, Category=text, Description=text, Value=number, Date=date]}}),
    
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"Year", "Month"}),
    #"Expanded Date" = Table.ExpandTableColumn(#"Removed Columns", "Date", 
                            {"Year", "Month", "Category", "Description", "Value","Date"})
in
    #"Expanded Date"