The 1000th burst was detected at 21:03 UT on September 21. It lasted for around 3 seconds, and consisted of a single large pulse of gamma-rays. It was automatically detected on board the observatory by the GBM and an alert was sent to the ground, that was then relayed to a worldwide team of astronomers in less than 15 seconds.
Originally, predictions indicated that we would need to wait for around 5 years before getting to the 1000th burst. However, due to excellent search routines implemented by the team of scientists who developed GBM, the rate of GRB detections has been significantly higher.
GRBs allow Fermi to see farther than any other class of object it detects and each GRB is a probe of the oldest and most violent explosions in the Universe. Every new one helps us better understand these interesting events.
