Frost in May is a fairly typical and nearly annual occurrence. However, a string of freezing nights along with scattered areas of snow is a little less typical. Still, it is not unprecedented.
The Fargo-Moorhead weather record from 1907 indicates 16 of the 31 nights during May were freezing. Eight nights were 25 degrees or colder. It snowed four separate times during the month, including a 2.9-inch snowfall May 13-14. In Grand Forks, 19 of the 31 mornings were freezing, with nine of those nights dropping to 25 degrees or colder. Snowfall records for that period for Grand Forks are sketchy, but the precipitation record suggests at least two rounds of measurable snow.
The year 1907 was over 100 years ago. The climate was a tad colder then. However, the anomalous conditions in the record book indicate nothing more than a pattern of Hudson Bay cold similar to the present pattern, but a bit more persistent.