Tohono Chul Park on the northwest side said the expected mass bloom for its night-blooming cereus will happen over two days this year.
"The blooms split, so half will be going this evening and half will go tomorrow," said Marcia Ring, the park's director of marketing and communications.
The park will be open from 6 p.m. to midnight Thursday and Friday.
Admission is $5 for the general public, members and kids under 12 are free.
The "Queen of the Night" cactus looks like a dry stick most of the year, but once a year it slowly opens at dusk to a palm-sized flower, which wilts within a few hours of sunrise. About 2,000 people visit the park during its annual bloom nights, according to Tohono Chul.
The park is at 7366 N. Paseo del Norte, near West Ina Road and North Oracle Road.
Free members, $5 general public, 12-and-under are free