Yes. And this is probably not Udemy specific. Sales tend to be lower during weekends since people are less likely to spend long hours on laptops, tablets, computers, etc. like they do during weekdays.
Most people will focus their weekend time on family, entertainment, resting, shopping, house chores, etc. So even though they are still connected, on average, they spend less time connected than they do on weekdays. This leads to lower sales of your online courses during weekends.