Please answer a question about the following article about Beyoncé:

In 2015 Beyoncé signed an open letter which the ONE Campaign had been collecting signatures for; the letter was addressed to Angela Merkel and Nkosazana Dlamini-Zuma, urging them to focus on women as they serve as the head of the G7 in Germany and the AU in South Africa respectively, which will start to set the priorities in development funding before a main UN summit in September 2015 that will establish new development goals for the generation.

When did Beyonce sign a letter for ONE Campaign?
2015