Delete rows based on cell value

Hey,
I'm in need of macro which could do the following:
1. With active sheet, check what is the value in cell (If .Cells(6, "U").Value = [number] in my case),
2. If value is 28 then check in column "O" for numbers 29, 30 and 31 and delete entire rows,
3. If value is 29 then delete rows with 30 and 31,
4. If value is 30 then delete rows with 31,
5. If value is 31 do nothing, abort the macro.
There will never be lesser value than 28 and greater than 31.
Any help is greatly appreciated!