"Thus saith the LORD; If ye can break my covenant of the day, and my covenant of the night, and that there should not be day and night in their season" (Jeremiah 33:20).
When the earth was first created, "darkness was upon the face of the deep" (Genesis 1:2). But then God allowed His light to penetrate that darkness and "divided the light from the darkness," whereupon He "called the light Day, and the darkness He called Night" (Genesis 1:4-5).
Ever since that "first day," there has been an unbroken succession of nights and days. With the slight exception of the "long day" in the time of Joshua (Joshua 10:13), these day/night periods have ever since been essentially 24 hours long, and our text assures us it will always be so.
However, the ratio of night and day in the 24-hour period does vary rather widely, with latitude and time of year. God also established the "seasons" during His week of creation (Genesis 1:14), as well as "years." This required the establishing of earth's annual orbit around the sun but also a significant angle between earth's own axis of daily rotation and its axis of orbital revolution.
Because of this combination of geometries, the nights are longer than the days for half a year, then shorter than the days for the other half-year. Twice during the year, of course, the days and nights are equal. These two days are the "equinox" (meaning "equal nights"). One is the vernal equinox, marking the beginning of spring; the other is the autumnal equinox, indicating the start of the fall season. That date, this year, happens to be today!
After the great flood, God assured Noah: "While the earth remaineth, seedtime and harvest, and cold and heat, and summer and winter, and day and night shall not cease" (Genesis 8:22). God's covenant of night and day "in their season" will continue in perfect stability as long as Earth endures. HMM