If you really zoom in, you can see the general pattern: it is highest on a Saturday, and low on Monday-Thursday, with Friday and Sunday in intermediate positions. Furthermore, the troughs get noticeably higher between November and December. Finally, the troughs got a big, sudden boost in March 2020 that takes several months to slowly "recover". What other behavior follows that pattern?

It's reflective of work-in-office patterns--IPv6 penetration is much lower in business settings than in residential (or mobile) settings.