Given the task definition and input, reply with output. Generate a correct and concise answer for the question based on the words in the context.

Context : The 2015 Singapore Grand Prix (formally known as the 2015 Formula 1 Singapore Airlines Singapore Grand Prix) was a Formula One motor race held on 20 September 2015 at the Marina Bay Street Circuit in Marina Bay, Singapore.
Question : When did 2015 Singapore Grand Prix occur?
20 September 2015