when did the test start?
July 16, 1945.
Trinity (nuclear test)
It was conducted by the United States Army at 5:29 a.m. on July 16, 1945, as part of the Manhattan Project. 
what is trinity nuclear test? It was the code name of the first detonation of a nuclear device.
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Trinity (nuclear test) [SEP] Introduction
A rehearsal was held on May 7, 1945, in which of high explosive spiked with radioactive isotopes were detonated. The Gadget's detonation released the explosive energy of about . Observers included Vannevar Bush, James Chadwick, James Conant, Thomas Farrell, Enrico Fermi, Richard Feynman, Leslie Groves, Robert Oppenheimer, Geoffrey Taylor, Richard Tolman and John von Neumann.