You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Martin_Luther

Context:
Martin Luther married Katharina von Bora, one of 12 nuns he had helped escape from the Nimbschen Cistercian convent in April 1523, when he arranged for them to be smuggled out in herring barrels. "Suddenly, and while I was occupied with far different thoughts," he wrote to Wenceslaus Link, "the Lord has plunged me into marriage." At the time of their marriage, Katharina was 26 years old and Luther was 41 years old.

Question:
When did Luther save the group of nuns from the convent?

April 1523