I believe that 13 is viewed as an unlucky number from a variety of sources, but Mark 14:12-26 tells us that 13 ultimately is very blessed for us.
Poor old Judas has been portrayed as a fall guy (predestined to do it), the good guy (based on later Gnostic teaching) and a bad guy. Whatever his exact motivation (possibly disappointment in the direction Jesus was taking), Judas did hand Jesus over. In other gospels we see has his hand in the till. I wonder what his initial fascination with Jesus was? Was he convicted like Peter was? Amazed like Nathaniel was? Who knows.
The institution of the Lord's Supper has made for centuries of debate! How often? What does 'is' mean? Is it a sacrament or an ordinance? A full meal or a symbolic act? Mark's presentation is shorter than the other Synoptics. What then can we say?
Firstly, the elements of the meal represent Jesus in some meaningful way. The bread is in some sense Jesus' body, broken to be shared. Likewise, the wine represents Jesus' blood, which is shed for many. Again, people argue over the nature of the many, but Jesus' forthcoming death was poured out (in his flagellation quite literally given how much he would have bled) for many. It wasn't an accident or pointless.
The word covenant is important - what Jesus was doing in celebrating a meal that represented the formation of a people for God was a new arrangement, a new deal with God. Hence, even though Judas gave Jesus away, a meal with 12 men representing what 12 tribes had experienced (and surpassing it) was with Jesus, who represented God - indeed who was God in the flesh.
These parallels alone suggest regular eating together with some words of remembrance form a central part of Christian identity, those for whom blood was shed. It's the new deal, and the real deal.