In Drupal 7, it’s easy to show calendar (ui.datepicker) date selector to user. (it’s not hard in Drupal 6, too - but in d6 you need to enable http://drupal.org/project/jquery_ui to get the calendar, I think)

You just write several lines of js code, and you get a calendar.
But things are quickly getting more complicated when you need to allow visitors to choose from dates that have some nodes “attached” to them only. E.g. if you have news view, and you don’t have news for 13 Nov 2011, there is no sense in allowing user to click on this date in calendar (I’m talking about the case when clicking on calendar redirects user to views page /news/2011/11/02 , where 2011/11/02 is a context filter for news nodes)

Here is the screenshot illustrating the desired functionality:

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