Why a happy season can be a hard one
Why do I feel low during a festival?
A festival narrows the range of feeling that is socially permitted. For most of the year nobody requires you to be visibly happy on a specific date. Diwali does, and it does so in front of an audience of relatives, neighbours and a phone full of other people's celebrations.
When the required feeling and the actual feeling do not match, the gap becomes a second problem sitting on top of the first. Now there is whatever was already wrong, plus the work of concealing it, plus the suspicion that something must be wrong with you for feeling this way during the good part of the year.
That third layer does most of the damage. The low mood is ordinary. The shame about the low mood is what keeps people from mentioning it to anyone.