On average, there are 13 days a year in which the afternoon temperatures in Fargo-Moorhead are 90 degrees or warmer. Some years have more, of course, and some years have less. This compares to the average of 10 days each winter with a high temperature below zero.
Both of these sets of days represent a kind of normal extreme. These are the sorts of days we expect to happen every summer and every winter, yet these are days that are clearly well above or below average on any given day.
The average high temperature on any given day is nothing more than an average and is not meant to suggest what the weather is supposed to be. In fact, there is a range of expectations and even the occasional radical outlier is to be expected. The majority of the summer days in the 90s usually occur in July and August, whereas the majority of subzero afternoons in winter occur in January and February.