Is there a Haskell library for dates?

Are Data.Time.Calendar and Data.Time.Format in the time library sufficient?

You can parse a string representation of a date and get the length of a month using gregorianMonthLength. Not sure about day of the week, though you could format the date as a string using a format that just displays the week day.

A quick Google search turns up this, which may be what you want. It lets you parse strings representing dates and extract information from them.