When does a weather day change? Why does Saturday’s forecast low occur on Sunday morning?

Dear Tom,
I know you’ve addressed this before, but I still get confused. When does a weather day change? Why does Saturday’s forecast low occur on Sunday morning?
— Lou Linneweh,  Carol Stream, Jim Eder, Mundelein
Dear Lou and Jim,
It can be confusing, but there is a difference between a day’s observed high and low temperatures and the forecast high and low.
Past weather is always cataloged on a standard time calendar day.
So all of the meteorological variables on a past day — the high, low, precipitation, wind, etc. — would have all occurred within the 24-hour period for the day in question.
In contrast, the high/low temperature forecast for a future date would be the expected high for the day in question and the low for the following morning.