I really don't feel that my technical ability varies from day to day. I'm not the greatest to begin with, but it's always pretty consistent. Now my improvising/creativity definitely has it's good and bad days. Also my tone (or my ear for it) changes a lot as well. One day I'll be thinking "damn, this sounds good, I'm never selling this amp/pedal/guitar!" and the next I can't get a good sound no matter what I do.