Most probably there are five days of objectives but the derby event stays open for longer. This has been done before also. I think what's missing this time is the "info" label that tells us how long the derby will be open.

Thanks.

Derby duration at least 15 days more. Otherwise 100 Daily Objectives are not possible.

But most task will be Tailsting.

Last Derby (this type) only 50 were possible without mech

How do I complete the 10-day entry when the mission lasts 5 days? 

The event ends on 21st June.