Edit: I just remembered seeing something similar related to rounding a floating point signal once. Maybe that is the reason behind it.
Edit 2: Peaks seem to fall on Saturdays, so maybe this corresponds to people turning on their computers/routers at home more often on the weekend.
It's reflective of work-in-office patterns--IPv6 penetration is much lower in business settings than in residential (or mobile) settings.
(At least as I've heard it explained before. But it could be a large country with high adoption like India swaying the statistics.)
I'm surprised that it falls back pretty equally on Fridays and Sundays, though. Maybe this is an artefact of the timezone they are operating in - some of the Friday traffic is actually Saturday, some of the Sunday traffic has gone to Saturday.
Other comments are pointing out it's much much easier to migrate mobile to ipv6 than it is to migrate desktop, so I'm thinking it's more likely this than the other possibilities.