I have unknowingly activated chat notification for Happy Hour.
Is there a way to deactivate it?
You need unregister yourself from event of Happy Hour.
I don't chat, but this section of the Chat FAQ seems relevant:
Can I mute someone?
Occasionally, you simply need to ignore someone - or a busy feed. To hide a user's posts temporarily, click on their avatar to see their user card …
… and select "hide posts". To see them again, reload the page. Or, you can ignore them permanently by clicking "ignore this user". You can manage your ignores on the preferences tab on your profile.
UPDATE: I have looked at the interface for specific Chat Room events. For the chat room, when you click the info hyperlink, you find data about the room and a "Register" button. Below is a collection of registered users. I suppose if you've registered for a room notification, that button will say unregister. Or the item with your information there will have some comparable function. (Or, I could be way off.)