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.