Awkwardly googling "how do you wish someone a happy Diwali" and hoping that the folks in India I deal with find my attempted well-wishing less stiff and insincere than it feels to me.
@fineursine that's an excellent point, and one that gave me a brief moment of panic that I hadn't considered it, but fortunately the entire office there is having a Diwali party tomorrow, then is on holiday the next two days, so I should be fine at least acknowledging it as a thing