Here's a run-down that should cover everything discussed, as people are getting heated over simple misunderstandings and opinions. Try to state opinions without berating other people ... it's really not necessary.
blood being taken: take it easy and do a light walk. It's important to maintain a constant workout schedule but not at the expense of your overall health. There is absolutely no reason to push it. Not eating and then getting blood drawn can cause problems with rigorous exercise, so a nice walk is a great idea.
diet: day-off of the normal diet. It's a good idea, if done correctly. A day off is less of a day off and more of a meal off. If you are on a strict diet, your body may have cravings because of what it used to be fed. You need to satisfy those cravings otherwise it may wear you down and you end up cheating during the normal diet days. Over time, those cravings should disappear, if you are eating correctly (getting all the nutrients, fat, protein, etc). Over the long term, if you are still having cravings, your diet may need an overhaul because you are probably missing something your body needs. I also find water is a great tool and curbing cravings.
If you've been on a constant diet than a cheat-meal should not affect you at all. Your body will find it's natural rhythm and automatically burn all the excess calories and will not be disrupted by one bad meal (within reason). It's the same in reverse. If you eat poorly all week and then decide to give your body a break by eating healthy, it has absolutely zero affect on your health ... it's like going to MacDonalds and ordering a Big Mac and large fries and a diet coke ... there's no point in the diet coke (unless for taste) ... you are only fooling your mind, not your body.
Note: I am not promoting cheat-days. I'm just saying that if you feel they are necessary, then it's not such a bad thing. I have my cheat-days once or twice a month, when a craving arises ... usually it's a sweet tooth and easily satisfied by a chocolate bar or ice cream bar ... that's my cheat meal. I can't even stand the taste of fast food anymore ... it all tastes like cardboard ... it was actually very disappointing when I discovered that, as I used to love it.
But, a diet should be a lifestyle, not a "diet". Whatever you are eating now should be a lifetime plan, not a short term rollercoaster plan.