Zoom in on that graph using the controls at the bottom, and you'll see a repeating pattern of crests and troughs, weekly. There's about a 5% difference between the crests and the troughs: the crests are hitting the 50% line or just below it, and the troughs are down around 45%.
The real question is, why are the crests so predictable? They're always on Saturdays; Sunday dips down a little below the crest, then Monday-Friday is down in the 45% range before the next Saturday jumps up to 50% again. (Fridays usually have a small rise, up to the 46-47% area).
My theory: mobile access rises on weekends. People are more often accessing Google services from their work computers Monday-Friday, but on Saturdays and Sundays most (not all) people are away from the office. Many of them will end up using smartphones rather than laptops for Internet access, for various reasons such as being outdoors. And since smartphones are nearly all using IPv6 these days, that means an uptick in IPv6 usage over the weekends.
It's not just mobile networking but residential ISPs in general have better IPv6 support. In the US, Comcast was one of the first big IPv6 deployments, in Europe CGNAT+IPv6 is common in many places.
Meanwhile corporate IT for business and education networks have less incentive to upgrade and typically lag behind in adoption in general.
I've been running full dual stack for >15 years now. It has become second nature by now and I'm slowly testing IPv6 mostly, but so far it's just easier to deliver dual-stack to all users instead of dealing with workarounds to make the last few non-IPv6 capable services work without native IPv4.
This matches what I've seen too. Mobile carriers have been way ahead on IPv6 - T-Mobile in the US has been IPv6-only with NAT64 for years. The weekend pattern is pretty much a smoking gun for mobile being the driver. It also explains why the Google metric (which skews consumer) looks so much better than the Wikipedia numbers someone else posted (35% IPv6) or the server-side adoption stats from Common Crawl.
Residential vs. business. If the graph was hourly and per country, you'd see the same rise every morning and drop every evening (likely by more than 5pp).
Your mention of per-country reminded me that Google is probably using UTC for those timestamps. 11:59 PM UTC is 8:59 AM the next day in Japan (UTC+9), so Japanese people getting online early Saturday morning would register as accessing Google services on "Friday" until the time hits 9:00 AM in their timezone. Likewise Korea (also UTC+9) until 9:00 AM local, China (UTC+8) until 8:00 AM local, Vietnam (UTC+7) until 7:00 AM local, and so on.
Which means that if Japan, Korea, China, Vietnam, and other east Asian countries have a higher IPv6 adoption in residential vs business ISPs, then their Saturday-morning Internet access is likely part of the 1-2pp bump on Fridays in this chart.
P.S. Also, none of Japan, Korea, China, or Vietnam use daylight savings time (very sensible of them), so their UTC offsets are the same year-round. So their Saturday-morning contributions to the Friday chart will not vary from month to month due to timezone slippage, because they will never gain nor lose an hour relative to UTC. It might vary a little with actual seasons, as the sun rises later or earlier... but so many people use alarms to get up at 6:00 AM no matter what the sun is doing, rather than rising with daylight, so the amount of early-morning Internet access in winter months is not going to change significantly compared to summer when the sun rises earlier.
The real question is, why are the crests so predictable? They're always on Saturdays; Sunday dips down a little below the crest, then Monday-Friday is down in the 45% range before the next Saturday jumps up to 50% again. (Fridays usually have a small rise, up to the 46-47% area).
My theory: mobile access rises on weekends. People are more often accessing Google services from their work computers Monday-Friday, but on Saturdays and Sundays most (not all) people are away from the office. Many of them will end up using smartphones rather than laptops for Internet access, for various reasons such as being outdoors. And since smartphones are nearly all using IPv6 these days, that means an uptick in IPv6 usage over the weekends.