I don't remember having to use 'mis' before each month when I learned Welsh in high school. It's 17 years ago though so could just be memory going me. Is it necessary?
It is better to use mis to avoid confusion with other meanings. The mis is often dropped with dates, though, as the context makes things clear. For example:
- Mawrth - Mars
- dydd Mawrth - Tuesday
- mis Mawrth - March
- yr ail o [fis Mawrth/Fawrth] - the 2nd of March
- medi - harvesting, reaping
- mis Medi - September