Answer by taking a quote from the following article:

Secretariat (March 30, 1970 - October 4, 1989) was an American Thoroughbred racehorse who, in 1973, became the first Triple Crown winner in 25 years. His record-breaking win in the Belmont Stakes, where he left the field 31 lengths behind him, is widely regarded as one of the greatest races of all time. During his racing career, he won five Eclipse Awards, including Horse of the Year honors at ages two and three. He was elected to the National Museum of Racing and Hall of Fame in 1974.

The 1973 Kentucky Derby on May 5 attracted a crowd of 134,476 to Churchill Downs, then the largest crowd in North American racing history. The bettors made the entry of Secretariat and Angle Light the 3-2 favorite, with Sham the second choice at 5-2. The start was marred when Twice a Prince reared in his stall, hitting Our Native, positioned next to him, and causing Sham to bang his head against the gate, loosening two teeth. Sham then broke poorly and cut himself, also bumping into Navajo. Secretariat avoided problems by breaking last from post position 10, then cut over to the rail. Early leader Shecky Greene set a reasonable pace, then gave way to Sham around the far turn. Secretariat came charging as they entered the stretch and battled with Sham down the stretch, finally pulling away to win by  2 1/2 lengths. Our Native finished eight lengths further back in third.  On his way to a still-standing track record of 1:59 2/5, Secretariat ran each quarter-mile segment faster than the one before it. The successive quarter-mile times were :25 1/5, :24, :23 4/5, :23 2/5, and :23. This means he was still accelerating as of the final quarter-mile of the race. No other horse had won the Derby in less than 2 minutes before, and it would not be accomplished again until Monarchos ran the race in 1:59.97 in 2001.  Sportswriter Mike Sullivan later said:  I was at Secretariat's Derby, in '73 ... That was ... just beauty, you know? He started in last place, which he tended to do. I was covering the second-place horse, which wound up being Sham. It looked like Sham's race going into the last turn, I think. The thing you have to understand is that Sham was fast, a beautiful horse. He would have had the Triple Crown in another year. And it just didn't seem like there could be anything faster than that. Everybody was watching him. It was over, more or less. And all of a sudden there was this, like, just a disruption in the corner of your eye, in your peripheral vision. And then before you could make out what it was, here Secretariat came. And then Secretariat had passed him. No one had ever seen anything run like that - a lot of the old guys said the same thing. It was like he was some other animal out there.

When did the horse enter the Kentucky Derby?
1973 Kentucky Derby on May 5