Can we say the Festival Onam, a psychological m treatment to counter depression ?

Yes, i think they can, but the problem is scepticism. People have to have a common belief of some kind to come together and celebrate like this. It's more motivating to believe you're doing something of cosmic importance, and many people may not buy into that, so they may not be as engaged as they need to be for it to help. I suppose they could take a "Festivus" - style approach.