Thanks for the positive reply!
Regarding your question, The first one about it being balanced. This is true! However you can simply tell all your workers to work as normal on said day, so not much of a festival happens, and festivals should only be able to happen over big events.
The second question raises the issue of it being a negative to have the festivals. This is both a negative as you allow some workers/other classes to have days off if you so choose. However the buffs it provides should make a difference if you allow festivals to be a big culture in your village (Making efforts and preparing for them) You also should be able to tick to make all workers not work or work, but maybe a drop down menu showing all the individual workers can provide an issue as you can choose exactly who gets the day off and who doesn’t.
Again, thanks for the positive feedback to my idea 
Wulfies