From my experience in internet businesses in developing countries, you tend to get quite strong weekly patterns as a lot of people only have access to computers at work, so usage tends to drop on the weekend.

Drops every Sunday¹. Employees of companies are using Tor to bypass company site blocks.

¹ https://metrics.torproject.org/users.html?graph=userstats-re...

No, I don't think so. Look at a bigger time frame:

https://metrics.torproject.org/users.html?graph=userstats-re...

There was a huge spike in Mid 2013, cause by a botnet that installed tor. Before there was almost not oscillation. After that there is quite a bit. I think there are many infected PCs at companies.

Do you still got a reference to news/blogs about that botnet install?

There is something form the official tor blog. https://blog.torproject.org/blog/how-to-handle-millions-new-...

Nice. That sounds like a good explanation.

It looks like a weekly cycle. Maybe some businesses use TOR during weekdays.