Welcome to the Board!
Just be aware that if you don't open the file on the first of the month, it won't work.
So if the first is on the weekend, and you do not open it on that day, the next day you do open it, it will not clear out the data.
Also, if you open it on the first of the month, and it clear out the data, and then you add new data, if you re-open again on that same day (the first of the month), it will clear the new data you just entered!
An alternative may be to have a cell store the last date that you cleared the data. Then, have Workbook_Open code that checks that date, and if the current month is different then the month in the stored date, then it will clear your data, update the stored date, and save your file.
That will make sure it clears out the data the first time you open it each month, and not again that same month (regardless of which day you open it on, in case you miss day 1).
That code would look something like this (I am using Z1 as the cell to store my date):
Code:
Private Sub Workbook_Open()
If Month(Date) <> Month(Sheets("Sheet1").Range("Z1")) Then
Sheets("Sheet1").Range("A1:B5").ClearContents
Sheets("Sheet1").Range("Z1") = Date
ActiveWorkbook.Save
End If
End Sub