Yesterday was Yoga X and, unfortunately, I was unable to do that exercise. My question is, do I replace the Legs & Back with Yoga X tonight, or just skip it (Yoga X) and do that on Sunday instead of a rest day?

you have a couple choices, everyone handles missed days different.

1- you can consider yesterday your rest day, and continue today with Yoga, etc...
2- you can just "forget" about yesterday, con't today with Yoga, then L&B, then kenpo, rest day...so now it'll take you 91 days.
3- you can double up and do yoga AND legs and back today (REEEEALLY not advisable tho, your legs will hate you). this would be more feasible with 2 workouts that dont use the same muscles.
4- or just skip the day entirely (meaning skipping yoga, which IMO = BAD!!). just dont make a habit of skipping workouts.

so, its really up to you. but just as i said in #4, dont make it a habit!

good luck!

Thank you!
I don't make it a habit of missing working outs. And I am actually on my second round of P90X. I'm not very good at dieting...and although my body has definition, I am definately not in the shape I was expecting. So, I am doing it again. Don't get me wrong, I LOVE LOVE LOVE P90X. I haven't lost or gained anything. Which I am very happy about. I am 5'4 and 1/2" and 120lbs. I'm just looking to maintain and get definition in areas that my other workouts weren't providing.

About missing the workout thats fine......and to be honest I imagine the one that people miss the most is the Yoga X because of how much it drags on.... I missed yesterday because The Dolphins game was on and Im just going to TRY and attempt doing Yoga and Kempo on Saturday. If that can't be done Im just using Thursday as my rest day and I'll go with doing Yoga on Sunday. Giving u a perspective of someone who missed too.....

Thank you for that...

I was happy that the World Series was on at night and not the daytime!!

Both Kempo and Yoga?

welcome to the forums! You'll find you'll get more definition if you eat according to the nutrition plan, but if doing what you're doing works for you, then keep bringing it!