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It was Epsom Derby in June 1931.
From the BBC website:
“He [ John Baird ] televised the first live transmission, of the Epsom Derby, in 1931.”
The book “Television: an international history of the formative years” gives the following account of the event:
“On Derby Day 1931, Baird Television's outside broadcast television van was taken to Epsom and parked opposite the winning post. It was connected by telephone line direct to the company's London studio, from where the signals were sent to the Brookman's Park transmitter.
The parade of the horses was seen, although the horses were not individually recognizable, together with shadowy images of moving people. [...]
After the transmission Baird said he was quite satisfied with the experiment. ‘This marks the entry of television into the outdoor field,’ he noted, ‘and should be the prelude to televising outdoor topical events.’ ”
BBC Historic Figures - John Logie BairdWikipedia - John Logie Baird"Visioneer: John Logie Baird and mechanical television" by Trevor Blake"Television: an international history of the formative years" by Richard W. Burns, pp 318-320