This can be an important question for any number of reasons. For example, if you are writing a work of history, you might need to be able to say authoritatively that something happened on a particular day of the week. Or, you might need an alibi for some reason, and being able to tell what day of the week a particular date was will help you to remember what you would have been doing on that particular day, not likely to have been at work on a Saturday, for example. Here is a link to a perpetual calendar, and there are others available on-line, too.
by viewing calender!!!