Calendar on Nokia Lumia 800 feels of similar taste as was on Nokia N9:
Being a kind of mis-managed person myself, I should be the last guy to review any calendar app at least, but even a n00b about Calendars like me can work out his things with Calendar on Windows Phone.
Metro UI in Windows Phone Calendar is kind of different than it would have been in contacts or Music app, making the first heading small and giving room to the dates and text itself.
I actually found it good to have extremely tiny mentions of events in month mode, just like we see it in real-life calendar. Animations are so smooth and lovely (slide up/down/left for changing the month and zoom out when tap on single date) that you always itch to make an event even if you don’t have any 😛
As expected from the social nature of the Windows Phone platform, Calendar on Windows Phone supports multiple calendar from multiple services just like Nokia N9 or Android etc. Each of the calendar either it be Facebook events, Windows Live Calendar, Google Calendar or Enterprise Exchange Calendar gets its own unique color (you can configure the same as per your choice from the eleven colors available).
When configuring an event from phone then you have two easy options, appointment in day/agenda section and to do in to-do section. You can always choose that in which service you wanna place this event. I am yet to check if the same calendar is open for any new service API/ app or not, but for popular services like Google Calendar or Facebook events, it works like a charm.