In French, putting 'le' before a day of the week implies that the event happens every Saturday, Sunday etc...
So, when talking about just one Saturday, the article 'the' isn't necessary.
Hope this helps (sorry I'm not very good at explaining)
Why, in this instance, does 'joué' not have to have an 's' on the end, as you would write with 'allés', 'arrivés' etc, to conform to the plural 'ils' ? I wrote "samedi dernier, ils ont joués ensemble" and was marked wrong.