so assuming you are not using any of the available web calendars (for example google calendar), I must say I don't know what you are talking about.
I understand it this way: when entering an event/entry you want some sort of widget that lets you pick a date, and if you look at the calendar, days with entries should be highlighted.
Now, how this will be implemented depends on the application. I wonder if there are extensions for webbrowsers that somehow detect a "date input field" and pop up that widget, but then where from should this extension know about your other appointments... so its back to the actual application:
What program are you talking about?