Much importance is placed on the success of each individual training session an athlete completes. Although the sum of these individual session is important, sometimes the emphasis placed on this 'micro view' of progress can be damaging. Negative reactions to a 'bad training day' can lead to an unrealistic view of progress, and the tenancy for an athlete to 'respond' rather than 'react'.
Instead, athletes should take a more 'macro' approach to the effectiveness of their commitment to a training program. The most meaningful data they can use is the general trend. A general trending upwards over time tells us a program is effective. Sure, there will be peaks and troughs, good days and bad days, but the general 'line of best fit' tells the story that we're interested in.
Almost anything that can be measured experiences these same peaks and troughs and irregularities of results. Stock market charts, daily temperatures, or the countless other measurable metrics of life.
So what's the take-away? An athlete should identify the factors causing a bad training session (and work to correct them), but, in the long run, their focus should be on patient long term gains. If the general trend is upwards, give yourself permission to have bad training days. Every data set has them. Refocus and make the next day better. Training not trending up? Then you have a problem and need to make a change.