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.