Are both sentences correct:
1.Parents' Day is on the seventh of December.
1a.The Parents' Day is on the seventh of December.
2.Mid-Autumn Festival is on the twelfth of September.
2a. The Mid-Autumn Festival is on the twelfth of September.
I'd use 1 and 2a.
Could you please give your reasons for the choice?
We rarely use the definite article with 'day'- Xmas day, Boxing Day. With 'festival' we sometimes omit it (harvest festival), but often include it- the summer festival, the village festival, etc.