To answer your question both legally and conventionally, you have provided very little information. Even so, I will try to address your query.
Weekly Off Days in Factories
1. If your factory operates 24/7, there is no question of a common weekly off day. There will be rotational weekly off days for groups of workers, known as staggered weekly off days.
2. If you are working six days a week, you can choose any day as your common weekly off. In both cases, you are not required to seek permission from the Factory Inspector, but you must notify him.
3. Normally, the common weekly off day is decided based on local customs, usage, market day, or the day when the electric supply is generally shut down. For example, in Pune, Maharashtra, Thursday is the common weekly off day. Therefore, if you choose a different day as the weekly holiday in Pune, it may not be effective. You need to determine the weekly off day in accordance with the prevailing practices in your factory area.
I hope this clarifies your questions.