Most "good day/bad day" experiences, in terms of performance are most likely due to dietary error.

Here are some tips to help:

• Eat similar foods at similar times every day -- your mind loves variety, but your body loves consistency;
• Remember the complete digestive cycle takes 3 days, so whatever you eat today will effect the next two days
• Your state matters -- Make sure you eat only when calm and relaxed.
  Eat slowly and avoid distraction.
• More isn't better -- more energy comes from increasing the nutritional richness of your food, not the quantity