Then [Jehu] wrote to them a second letter, saying, “If you are on my side, and if you are ready to obey me, take the heads of your master's sons and come to me at Jezreel tomorrow at this time.” Now the king's sons, seventy persons, were with the great men of the city, who were bringing them up.
2 Kings 10:6

The elders of Samaria were already terrified of Jehu. Giving them only 24 hours to act was a great tactic because it didn't give them time to coordinate and plot against him. It made them parties to his conspiracy and committed them to his side without allowing hesitation.