Yahoo! calendar is like a special book where you can write down important dates, like when your friend's birthday is or when you have to go to the dentist. You can also set reminders so you don't forget about these dates. It's like having a very organized friend who remembers everything for you! Plus, you can access your calendar from your computer or phone, so you can always check what you have planned. So, it's like your own personalized schedule that helps you remember all the important events in your life.