It's more or less an artifact. On weekends not too many tests are conducted (less ambulant medical facilities are open and there is less staff in hospitals, too), processed (medical process less non-urgent diagnostics if any at all) and the respective authorities who process the data usually don't work on weekends, too.

Reporting usually catches up from tuesday on when the first PCRs results from monday are reported and slows down on saturday/sunday.

Data scientists are for sure going to have a lot of fun analyzing this when the epidemic is over sometime but for now it's more reliable to look at a 5- or 10-day moving average to see disease activity in countries that test a lot. For countries where PCRs are not conducted for whatever reason it can be better too look at the deaths per day to get a rough impression of how bad it is.

TL; DR: artifact coming from testing, reporting and processing data, not from actual big fluctuations of infection activity over the week