About 120 contestants are expected to enter for a crack at a prize pool in excess of $30,000.
Many contestants spend thousands of dollars on their suits, dresses and headwear for the event, with designers kept busy with custom orders in the run-up.
However, those keen for a more laid-back experience can take their shorts and jandals to the Beach, Beats and BBQ party, which sees an artificial beach set up in the infield.
Auckland Racing Club chief executive Cameron George said the social aspect of the day was a key reason people kept coming back, year after year.
"It's a day that we have something for everybody, there's a lot of kids' entertainment and activities, and a good relaxing feel in the infield.
"We have our Fashion Village for the people that love fashion..
"People get dressed up, and others come in a relaxed mode after Christmas."
But the real highlight will be the racing, and gates open at 10am for the first of 10 races at 12.55pm. Mr George said the centrepiece would be the $200,000 Zabeel Classic.
"That's a very, very strong field in terms of quality.
"The likes of Survived which is favourite - if people want to see the best horses in New Zealand race, Ellerslie will certainly show them that."
• Pirongia Races. Alexandra Racing Club, Pirongia. First race at 11.30am.
• Katchafire and Kora Summer Tour. Waihi Beach Hotel, Waihi, 7pm.
• Boxing Day on The Peak concert. Te Mata Peak, Havelock North. 3pm to
midnight.
• Wellington Salon Champs and Demolition Derby. Wellington Speedway, Upper Hutt. 7pm.