How Come I Forgot?!

As a secretary in my faculty Student Council,,
every time we wanna hold our meeting every 3 months,,
the chairman always ask the vice, the treasurer, and the secretary for a day that we all can come to the meeting...

This October,, the meeting must be held and as usual I was asked for the perfect day...
I answered :
I have no class on Monday, Wednesday, and Saturday
Tuesday = available from 3:00 PM
Thursday = available from 1:00 PM
Friday = full...

So I was asked again for meeting on October 31 via SMS..
And I smoothly replied : "Yes, I can.."
I completely forgot that I have retreat on that day...

This morning,, suddenly I remembered that important event and Jarkom has been sent... (Jeng jeng... Back sound played..)
For your information,, I'm one of that retreat committees....
I directly sent message to the chairman to say that I'm so so sorry... (I really feel guilty for that..)
Until now,, I don't get the reply yet..
I hope he doesn't get mad of me.. >_<

The big question in my mind is "How come I forgot?!"