My initial hunch would be August 1910. I attached a screenshot of the lowest minimums from WA that month. That was definitely a verifiable, historic cold snap for August. As we had discussed previously, this is the same cold wave that dropped Bowen, Montana to 5 degrees for the US monthly August record low.
Off topic a little, but 1910 was sure an interesting weather year as far as off season temperatures go.
The March 1910 heat wave was possibly the most impressive March heatwave to ever hit the Plains and Midwest.
Some impressive readings (especially for March):
100 at Hugoton Kansas
89 At Lapeer Michigan
88 at Montevideo Minnesota
88 at Miles City Montana
101 at Grant Nebraska
90 at Edmore 1NW North Dakota
88 At Kennewick Washington (it seems that Chris discounted this reading though?).
May 1910 had a really impressive heat wave in much of Utah and Nevada.
108 at St George Utah
103 at Richfield Utah (the second hottest temperature ever recorded there in any month).
August 1910 was perhaps the most impressive summer cold snap anywhere in the United States with places like Bowen and Hegben Dam recording 5 degrees (3 degrees colder than the May record and only one degree from the September record).
December 1910 had another heat wave in the Southwest with temperatures soaring to a reported 91 at Las Vegas, but this one seems dubious.
If there were more weather stations in the US in 1910 there would be a lot more records set in all of the months mentioned above.
Coldest temperature thus far in 2018: -26 on 2/21
Warmest temperature thus far in 2018: 99 on 7/8 (All time record high)
Precip thus far in 2018: 11.01 inches
Snowfall thus far in 2018: 38.7 inches
Last frost of early summer: 7/1
First frost of late summer: 8/29
Last snow of late spring: 5/1
First snow of early fall: 10/6