what preparation was take for the unexpected murder game invitation at little paddocks

In the opening pages, a murder is announced in the newspaper Gazette in the dizzy town of Chipping Cleghorn. Someone must be putting an invitation for a game in the paper. Naturally, a murderer would not place an ad in the paper announcing their murder, right? However dangerous as it might be, it sparks the interest of the people in town. With a specific date and time announced in the paper, the neighbors arrive at the home of Miss Blacklock ("Little Paddocks"). At 6:30, the lights cut out. After gunshots ring, the people at Little Paddocks quickly realize that announcement was NOT a game. When the lightscome on, Miss Blacklock has an ear grazed by a bullet, and an unknown man lays breathless on the floor with a gun next to him.