The calendar on contacts is a great way to keep track of all events related to your clients. This can include viewings done, calls made and future reminders. These activity types can be configured by your administrator in settings.
Open any contact, click on the diary link, and you'll be able to access a calendar specific to that contact. You can switch between an activity timeline (showing all your calendar entries) and a typical calendar view:
From here you can add new activities including notes (which can also be pinned to the top for easy reference). Any events you capture here will be saved against the contact, and will also reflect on your main calendar.
When adding events, you can filter the event type and add a date and description. You can also link relevant listings (e.g. a buyer will view a specific property). Linking listings on contact events, will also reflect those items in the specific listing calendar.