For a while I tried to use org-mode to schedule appointments and keep track of my calendar. But eventually I gave up. I never found an effective way to coordinate with others. I still use Emacs to mark non-calendar-blocking events like market half days. My phone can easily keep track of regular events like birthdays. But recurrent events with more complicated rules are a pain. For example, crude oil futures expire the third business day before the 25th day of the month if the 25th is a business day, or the fourth prior day if the 25th is a holiday.