"It was really hot the day before you were born. Your Dad and I went to the Tastee Freeze in the evening the day before." I don't remember if she told me if she had a plain vanilla cone or what.
It is difficult to quantify "really hot" and the genealogy gods tell us not to believe anyone when they say anything. But sometimes it is difficult to find evidence to prove what the weather was on a certain day.
And so I went to WolframAlpha and searched for:
weather carthage illinois daybeforebirthdate
where "daybeforebirthdate" is the day before my actual date of birth. And sure enough, there was the weather. And sure enough, Mom was right. The few screen shots included have the date omitted, but there's enough information there to see what was obtained.
There was a high temperature of 92 that day, which did strike me as being warm for that time of year--but it was possible I was wrong.
Sure enough, I wasn't imagining that 92 was a little warmer than usual for that day. Of course, I don't remember the year I was born, but I do remember the weather on my birthday in years after my birth. The historical temperature data included on the site told me that the average high was only 83--nine degrees cooler than the high the actual day before I was born.
WolframAlpha may be able to give you some insight into the weather on a day of interest in your personal or family history. It's pulling the weather statistics from some national database is my hypothesis and you probably won't get weather data this specific for 1235.
Whether your mom is right about the weather in your case is another story entirely.