((Sorry for the lack of posts. I've been sick with strep throat and have been in bed.))
Routine is very well known to be something that people with Autistic Spectrum Disorders can be obsessive over. We like things to be just so and hate to have any routine interrupted.
When I was a kid, my mom would have to warn me over and over again before she would change the activity I was doing. If I was playing on the computer, she would come down two or three times before dinner to warn me that we were going to be having dinner at a particular time and that I would have to get off the computer. When I was in classes at school for kids with Asperger's Syndrome, there were schedules and charts everywhere and the teachers were constantly writing out what activities were going on when on the board. This made things very safe. The world seemed very controlled and eased a lot of my worries.
As I've gotten older, I've relaxed a lot in my routines, but I still need warning if something is going to be changed. Several times a class has not met for a day and I didn't get the email about it so I became nervous that something had happened, they they had gone somewhere for class and I wasn't told. I'll often check a piece of paper over and over again to make sure that I have times exactly right, that I know precisely where I'm going or else I panic.
I no longer throw fits when my schedule is interrupted, but it's still a very important safety feature in life. A predictable schedule where I know day to day and week to week what is going on allows me to relax and worry about things that actually matter instead of fearing the possibilities.