Hi Wim,
Yes, this is possible, but note that there are 66 games to play, which is not divisible by 5, so this leaves one day where there is only one game to be played. Here is a schedule where the players are labelled A to L.
(B C) (---) (---) (---) (---)
(G K) (E F) (H L) (D I) (A J)
(D J) (G L) (B E) (C F) (H I)
(B K) (I J) (A C) (E H) (F L)
(G H) (J K) (D F) (B L) (A E)
(E G) (B I) (F K) (C J) (A D)
(D H) (A K) (J L) (F G) (C I)
(A F) (D G) (E I) (C L) (B H)
(C D) (A L) (B G) (H J) (E K)
(A I) (C E) (D L) (H K) (F J)
(F I) (D K) (E J) (A B) (C G)
(B J) (C H) (G I) (D E) (K L)
(E L) (B F) (A H) (I K) (G J)
(I L) (F H) (C K) (A G) (B D)
Ian
