The hammer rang once. A spark jumped off the anvil and did not go out. It waited. They followed.

It rolled across the Green and stopped on the step of the painted wagon. On the roof, the trunk had seven locks. One was already open. The second one glinted in the sun.

The wagon step had been mended with a fresh plank. A tiny hammer was scratched into the wood.

Danya folded his arms. Last time he had found the reason behind the trick. This time he wanted to catch them out.

A tall performer came down the wagon steps with a calendar rolled under one arm. The calendar had numbers on it, but no days of the week. This was Wendel.

"Tell me your birthday this year," said Wendel to the crowd. "The date, not the day."

"Twenty-third of October," said Beto.

"A Friday," said Wendel, without looking at anything.

Beto had a paper calendar in his pocket. He checked. Friday.

"Nineteenth of June," called Mrs Patel from the back.

"Friday too," said Wendel.

Beto checked again. Friday.

The crowd clapped. Danya did not. "Lucky guesses," he said under his breath.

Danya put up his hand. He had a plan.

"Twenty-ninth of February," he said.

Wendel smiled. "This year has no twenty-ninth of February. It isn't a leap year. You can't catch me with a day that isn't there."

A few people laughed. Danya went red. He had been so busy trying to catch Wendel that he had not checked his own date.

After the show, Marlow, the troupe's stage manager, sat on the wagon step in his velvet waistcoat.

"How does Wendel do it?" asked Danya.

"I will not tell you," said Marlow kindly. "But write down what you got. Every date tonight that came out a Saturday."

Danya went back through the show in his head. People had shouted dates all evening. He wrote down the ones that were Saturdays.

4 April. 6 June. 8 August. 10 October. 12 December.

"Look at them," said Marlow. "What do they have in common?"

Danya stared. "Four four. Six six. Eight eight. Ten ten. Twelve twelve. The day and the month are the same number."

"And this year," said Wendel from the doorway, "every one of them is a Saturday. Find the nearest anchor."

A quiet member of the troupe was packing up props. This was Modo. Modo drew a big circle on the chalkboard and put seven marks round it.

"A week is a clock," said Modo. "Seven marks, round and round. You only care where you land."

"Count thirteen days," said Modo. "Round the clock once, that's seven. Then six more. Thirteen on a clock of seven is six."

Danya looked at Beto's birthday. Twenty-third of October. The nearest anchor was the tenth: a Saturday. Twenty-three take away ten is thirteen. Take away seven. Six left.

"Saturday, and six more days," he said slowly. "Sunday, Monday, Tuesday, Wednesday, Thursday, Friday."

"Friday," said Modo. "Take away the sevens."

The next evening, the troupe asked Danya to do the Reveal.

He stood at the chalkboard. "Give me a date this year."

"Christmas," shouted someone.

Danya found the anchor. "Twelfth of December is a Saturday," he said. "Christmas is thirteen days later. Take away seven, six left. Saturday and six more days is a Friday."

He drew the clock and showed them, mark by mark. Then the crowd checked their phones. Friday.

That night, the second lock on the trunk clicked open.

Back on the Green

The spark dimmed to a warm dot and settled back on the anvil. The Green was just the Green again.

At breakfast, Beto's gran said her birthday was on the eighth of August.

"A Saturday," said Danya. "Party in the afternoon?"