off day noun

1. a day when you do not do things as well as usual
   • ● Even the best players sometimes have an off day.

[TahlilGaran] English Dictionary ▲

off day
n. phr. A period when one is not functioning at his or her best; a period of weakness.
The champion was obviously having an off day; otherwise she would have been able to defeat her opponent.
Antonym: DAY OFF.

[TahlilGaran] English Idioms Dictionary ▲