The fictional date.
According to the IMDB goofs section for Four Guns to the Border:
"When Jay Silverheels throws his knife into the calendar for June 1881, it shows June having 31 days. When he repeats the throw a few moments later, the hole made by the first throw has disappeared and June now has 30 days."
June always has 30 days in real life. In AD 1881 June 1 was a Wednesday and June 30 was a Thursday. Since I didn't see that part of the movie I can't say what weekday June 1 was in the movie calendar(s), but I would guess the probability that it wasn't a Wednesday would be rather high.
In real history, June 1881 would have a decent probability of having having a small band of hostile Apaches near the Mexican border. The western Apaches all settled down on reservations, willingly or otherwise, in 1872-73, and from then on thousands of Apaches, the vast majority, were at peace. But there were a number of outbreaks of Apache groups from the reservations in the next few years, so that at any moment from 1873 to 1886 the number of hostile Apache men, women, and children on the loose might be anywhere from zero to over five hundred.
Not that the makers of westerns were ever careful to restrict their depictions of peace or war with various tribes or nations to the years when there actually was peace or war with those groups.