Eat the frog
Eat the frog means doing the hardest or most avoided task first, before the day fills up. The phrase comes from Brian Tracy's 2001 book, and the frog is whatever you are most likely to put off.
Also called: eat that frog
Where it goes wrong
Hardest and most important are not the same thing, and the rule conflates them. Spending your best hour on the task you dread most is only correct when dread and value happen to coincide.
Diarized schedules the work you already keep in a to-do app into the calendar you already use, and shows you why every block landed where it did.