The sense of the vernal commencement does indeed refer to the onset of spring. It was considered the time when nature came alive. It was a time when birds would build nests, vegetation would grow, flowers would begin to bloom at varying rates, and insects would begin to populate the world for a new season. This is a time of year that was often celebrated by pagan religions.
Also known as the Vernal Equinox, March 20 [was] the first day of Spring 2009. The Spring Equinox is a day that symbolizes dawn, youth, birth, and rejuvenation.
With this information in mind, I have constructed the following sentence:
The villagers collected flowers and greenery to decorate the square in preparation for the celebration of the vernal commencement, which was a major holiday in that part of the country.