Software to calculate accurate dates and times for equinoxes, solstices and the Celtic cross-quarter days. The cross-quarter days are those midway between the equinoxes and solstices. For example, Imbolc is midway between the northern winter solstice and the northern spring equinox (a.k.a. the northern vernal equinox), and Beltane is midway between the northern spring equinox and the northern summer solstice.