The in-game calendar has a lot of quirks and is missing a couple of features, but that doesn't mean it's entirely useless. The calendar is a wonderful tool for alerting your friends that you'll be online at a certain time and that there's a quest or instance you would like to do. You can create events, invite anyone, and then check to see how many have signed up.
Using the calendar, you can rid your quest log of all those built-up group quests that no one ever seems to want to do with you. By giving plenty of time advance notice, your friends will have time to gather the same quests you have, and you'll all be able to reap the rewards together.
Creating an event is easy. All you do is right click on the day you want the event to be on. I suggest at least a week in advance, but no more than three weeks notice. A week gives them time to catch up to you, two weeks gives them time to do it themselves, and in three weeks they'll forget you even planned a day.
The layout is simple and the options are practical.
For a name, you should choose something that could be understood at a quick glance. "Icecrown group quests" would be perfect. "Blackrock instance clearing" would also work.
is server time.
Remember when choosing a time that it's the time that will show up to all other players who are invited, so enter it as server time so no one gets confused.
For group quests, you should enter the name of the quests in the Description field. If it's an instance and you have quests for it, put them there also. This way players can look them up on thottbot or wowhead and get them for themselves.
Let me bring up that you can right click on an invited player's name and promote them to moderator. This allows them to recruit their own friends to join you. It's very worthwhile to promote your trusted friends to moderators for any of your events that you don't think you'll have enough people for.
Try to only have one or two events waiting on the calendar at any time. Players can be overwhelmed when the calendar is used for raiding, instancing, world events, and heroic resets. Too many future plans, and they could become confused and miss out.
The calendar and event planner are simple but effective tools. Play around with them to become more familiar before setting up regular events.