I'm sure there are different schools of thought on this, so any pro's or coaches please chime in...That being said, I'd say whenever you feel like.  There are some areas that begin skills camps around August, because typically racing begins around September.  I for one think the more comfortable you are with 'cross specific skills the faster you'll be, and you can only be comfortable with plenty of practice. 

I had really good success last season loosely basing my training on The Time Crunched Cyclist program. I started hitting the intervals hard around 2 months before the season and continued with that through most of the season.

As a clyde I made the decision awhile ago that I would focus on short / high output races (CX, Enduro, Crit). I now use almost every ride to focus on that style of riding. On rides I try to focus on 60 min rides w/ lots of rolling hills and sprints. I really try to emulate the demands of CX by getting my heart rate up and down & I ride in big gears to simulate the extra effort of CX.

I'm also a big fan of www.bikejames.com

Jeff ~

Loosely based = I'm not hard core and I don't have a power trap so I did some research on my own for HR training. 

One way that worked for me while getting started (getting a feel for how my body responds at different levels) was to find something with a loop ~ High School running track,  corporate parking lot, park, etc...  I would go out to a place and figure out my sprint time between points and then I would start my 4 sets of 8 sprints using the points as referances (no timer / HR monitor / etc).  For example:  HS running tracks work great (when no one is there).  For example: Sprint the whole lap and spin for 1/2 lap - then sprint again...  I liked doing this because I didn't have to think about it or look at a timer / HR monitor / etc.

After a couploe weeks you'll start to get a feel for how your body responds and then start using your HR monitor more and maybe a Gym Boss.

I was able to follow the Time Crunched program pretty close using this ~ I just didn't spend all the time on power files, tracking heart rate, etc... Even without that I went from a top 10 to a top 5 rider.

I'm a strong beliver in total body fitness ~ which is why I suggested using all of Bikejames stretching and strength routines.  I found the TCC program got me really fast ~ but I struggled with hip and back issues toward the end of the season because I let a bunch of other stuff go ~ CX is hard on the body ~ so get a day in of plyo's w/ focus on those weak spots like hips.