I presume your question is related to "Festival Offs" (and not festival 'leaves')
The best way to deal with such a situation is to have a few "common" festival offs (including National Holidays) and a few as "restricted" festival offs wherein an employee can chose his own bouquet of festival offs. However, under this system a few things should be kept in mind e.g. what percentage is common and and how many can be restricted. Secondly, during 'restricted' holidays, how many of the employees/ staff are present department wise and whether that number is optimal to meet the department's productivity expectations?
I hope I am able to clarify your doubts.