Yes, it's a "dative of apposition" - not a phrase you hear every day! It's tending to die out in German these days - you probably hear "am Montag den zehnten Februar" nowadays at least as frequently as the theoretically correct "am Montag dem zehnten". Language change in contemporary German has proved lucrative for Bastian Sick among others! ('Der Dativ ist dem Genitiv sein Tod'). I don't think Germans get quite as outraged as (UK) English speakers do at every development in the language though!