Is it possible to highlight or grey a whole day in calendar view?

No, the calendar is remarkably un-customizable. I would love more functionality in altering the appearance of the basic calendar, especially the inclusion of a week-at-a-glance view like paper datebooks. Palm had this grid view on the Treo 600, and after dropping it in later models Pimlico Software (who licensed the original calendar to Palm) provided an aftermarket calendar program called DateBk. Highly customizable, and a prime reason I kept using my Treo for so long.

Adding the ability to block out or color entire days in the monthly calendar view would be a nice feature.

Thank you very much Rambo. One would think it would have it as my old flip phone did.