Zon = O.K. (short for "Zondag")

Maandelijks should be "Ma" (short for "Maandag")

Di = O.K. (short for "Dinsdag")

Rood should be "Wo" (short for "Woensdag")

Do = O.K. (short for "Donderdag")

Vrienden should be "Vr" (short for "Vrijdag")

Statistieken should be "Za" (short for "Zaterdag")

I think it would be best to be consistent in number of characters when using abbreviations.

either [Zo,Ma,Di,Wo,Do,Vr,Za] or [Zon,Maa,Din,Woe,Don,vri,Zat]