you will get varied responses on this topic,
I have tried every split known to man with limited sucess, I now go by how my body is reacting to training and diet. I dont belive in mon/chest tue/legs and so on, the body dosent know what day is what. You have a lot of limiting factors, you may have been stressed at work and missed a meal and but you decide to go train legs anyway, chances are you will not be able to perform at your best, I personally would take the day off rest,eat and hit it the next day.
Every routine i have tried that is set in stone has led me to overtrain especially during contest prep, My thought is it is better to be 10% undertrained then 10% overtrained! Granted you will have some days you need to motivate yourself to get to the gym but they should be few and far between. I remember reading Dorian Yates book, in it he says he would get to the gym and do a set or 2 and if he didnt feel like he could perform to his abilities he would walk out of the gym with no guilt what so ever and return the next day with a chip on his shoulder and train even harder! This made a lot of sense to me when i read it and have done it many times.
hope this helps