iOS: Fantastical is one of our favourite calendars on iOS and it now boasts a handy new drafts feature. As its name implies, this allows you to save events as drafts so you can dig through your appointments to reference other items. Here's how it works.
When you create a new event, drag down on the screen to enter the drafts mode. You can now look through your calendar to see if your new events is conflicting with something. You can create multiple drafts of calendar appointments or reminders here, so you can easily make sure you're not double-booking yourself.
Fantastical 2 ($US4.99) [iTunes App Store]