Funny thing is despite being the year without a summer there were actually short but pretty intense heatwaves with temperatures reaching over 100 degrees with very high humidity.
These heatwaves only lasted a couple days then it'd go right back to subfreezing cold and snow. Most days in June July and August were cold, subfreezing, snowy, and icy. The very few days that weren't though were extremely hot and humid. Like I said over 100 degrees with heat indexes probably (can't be sure though) 120-130+.
Of course the upper Midwest and much of the northeast is known for wild temperature swings. Temperatures in the upper Midwest and much of the northeast are usually rather cool even on the summer. However you can have temperatures well over 100 with extreme high humidity pushing heat indexes to 120,130, 140 or even 150. Heat indexes not experienced even on the worst days in the deep South, the gulf coast and Florida.
Bit then you can also have summer days with subfreezing cold, snow and ice in the summer, even in July and August in the upper Midwest and parts of the northeast.
There's times in the upper Midwest and parts of the northeast where you can go from subfreezing, ice and snow to 100°+ heat with a 130°+ heat index in a couple days, then back down to subfreezing cold and snow in another couple days, back up to 100°( heat with a 130°+ heat index on another couple days and back down to subfreezing cold and snow again in another couple days in July and August.
Of course 1816 took this to the extreme, the vast majority of days being subfreezing, snowy and icy with a couple days of 100°+ days with extreme high humidity, each sandwiched between about 3 weeks of subfreezing cold and snow.