Planner is made for your more accomodation in working with Calendar option.
As you can see, Planner shows you all day time, from the earliest morning to the very night. If you wish, you are able to choose needed Active time (for example from 8 a.m. to 11 p.m.) for showing in Planner with choosed colour, or simply only Active time. It can be choosed in Calendar settings.
You can choose for viewing a day, work week (five days), a week (seven days) and a month. There also are month's calendars on the right side of the Planner, that shows the days, when you have any notification by marking them with black colour. And, of course, you may choose the time scale. It can be shown with every 5 minutes, 10, 15, 30 or an hour.
To see current event details press 
button on the toolbar. to choose another colour for the very window, use Calendar settings.
For your more comfortability in working with your Organiser, you now are able to set To-Do list to be viewed in the Planner. If it is done, you may work with your To-Do list: read it from the phone, add new tasks and edit existing ones, delete them and write changes back to phone.
See also:
Working with Oxygen Phone Manager for Nokia phones