Hi Blog,
Having a calendar which record down our meetings and appointment, even our friends' birthday, is an essential tool for busy adult like me :)
When I was young, remember someone birthday is not a problem because you only have a handful of friends to get along with. But when I got older, I tend to forget some of latest friends' birthdays, meeting up or appointments; there are so many things you need to do and attend.
Lucky I have a phone which helps me to track down everything. Personally I have not used or tried out many calendar apps, beside our iPhone calendar, I have one which I like. It is called "CalenMob".