Litha is the season of expansion, when the crops burgeon forth. We forget winters cares and spend our days basking under the brilliant light. The Summer Solstice brings us the longest day of the year - the zenith of the Sun King, and also His death as the Holly King dethrones him and takes reign over the now waning year. From now until Yule, the light will fade.
More info on correspondences for this Sabbat can be found HERE.