I realize this post is a year old but hopefully my 2 cents will help. I'm a holiday sales associate.
As I look at my schedule on "Insite," I can see every date from now until January 4, 2014, which I'm supposing is the end of the holiday season. I looked at every single date except for one I can't do because of a prior commitment. I was only able to pick up ten shifts, for a total of about 80 hours. I have a shift today that I didn't get through "Insite," because I manager approached me last week when I was working and asked me if I wanted it.
Holiday associates are behind full and part time permanent in the pecking order and those folks see the schedule before us, which is fair, they've been there longer. What I also notice is that on the key sale days (eg "Black Friday") you will find shifts, especially since my store opens at 8pm on Thanksgiving.
The other thing is that people will take off and then you can cover the shifts that become available. The trick is to monitor the schedule every day.