eukairos describes a fitting occasion, whether a day for an event or help received at the needed time.

The two verses use the adjective for what is fitting to a particular occasion: a day on which Herod holds a banquet and help found in connection with mercy and grace.

1. Fitting to a particular occasion · dominant

   In Mark 6:21, the genitive singular feminine form occurs with “day” in the clause introducing Herod's birthday banquet; the APB renders the clause “An opportune day arrived.” In Hebrews 4:16, the accusative singular feminine form occurs with “help” after receiving mercy and finding grace; the APB renders “grace for opportune help.”

   Mark 6:21Hebrews 4:16

In the Anselm Project Bible: The APB uses “opportune” with both “day” in Mark 6:21 and “help” in Hebrews 4:16. These phrases show the verse-level presentation rather than a fixed English equivalent.