"Pioneer Day commemorates the arrival of the first group of Mormon pioneers to the Salt Lake Valley, on July 24, 1847. In Utah it is an official state holiday, and the associated celebration, including a parade, is referred to as Days of '47. It is a time for recognizing all people who have contributed to building the state, regardless of religion or background."
To learn more about its significance, visit the link below from which the above quote is taken.
https://www.churchofjesuschrist.org/study/new-era/2010/07/to-the-point/what-is-the-significance-of-pioneer-day-is-it-celebrated-all-over-the-church?lang=eng