Actually, Hanumanji escaped the lord Shani from the Ravana's prison. Due to this, lord Shani pleased with Hanumanji and gave boon that whoever will worship lord Hanumanji then there will be no effect of lord Shani on him/her
According to this question and a comment posted here, states that
Rahu and Ketu take up Tuesdays and Saturdays respectively
And as I remember (Please correct me if I am wrong) there was an incident of HanumanJi with Rahu, when HanumanJi tried to eat the Sun (I don't have the exact references for this).
So it could be linked to that.
I found the ...