Read the text about an Italian tradition:
The 'Palio di Siena' is a historic horse race held twice a year, on July 2nd and August 16th, in the Piazza del Campo in Siena, Italy. Ten horses and riders, representing ten of the seventeen city wards (contrade), compete fiercely in a bareback race around the square. The event is much more than a race; it is deeply rooted in the history and identity of Siena, involving elaborate parades, blessings of the horses, and passionate rivalry among the contrade, attracting thousands of spectators.
Based on the passage, how often does the 'Palio di Siena' horse race take place each year?
A) Once a year, in July.
B) Three times a year, in different months.
C) Only when there is a special occasion.
D) Twice a year, on July 2nd and August 16th.
E) Four times a year, to represent the seasons.