There are several factors here.
These are very big drives. There's a good reason it's generally recommended not to use RAID5 (or other parity RAIDs) with extremely large arrays: they take a long time to rebuild. The rebuild process puts a lot more stress on all remaining drives than usual, leaving a very high chance to trigger another failure. RAID 6 is better than 5, but it can still have issues as you see here.
These drives are probably from the same batch. Normally it's recommended to get drives from different batches, or even different ages, to stagger likely failures as they occur.
These particular drives are notorious for exceedingly high failure rates. Two failures out of seven on consecutive days isn't even particularly unlikely at this age for these drives!
Going forward, what you should do is:
Maintain backups. Remember that RAID is imperfect; critical data should always have offsite backups.
Try to purchase drives that aren't all from the same batch. That reduces the likelihood of consecutive failures.
Avoid very large arrays. Better to split into multiple logical volumes if possible, and maybe use smaller physical drives. Otherwise, mirrored RAID is better (faster rebuild) at the cost of more unusable physical space.