My periods are extremely regular for the most part, but maybe twice a year, i will have one that's either several days later or earlier than usual/expected. is it normal to have one or two periods a year that are a bit off?

Sure: Many women have occasional variation of their mucus discharge, some spotting between periods, a heavier period, a lighter period, a longer or shorter period or cycle, or even a missed period. Having one or two of these symptoms doesn’t necessarily mean there is a medical problem, but a visit to one’s primary care or Ob-Gyn doctor is a good idea if symptoms are persistent, recurring, or worrisome.