Summer Solstice 2013: Why It's the First Day of Summer

This year's summer solstice will be followed by the year's largest supermoon.

A Russian neo-pagan supreme priest, right, beats a tambourine while his followers dance around a bonfire to celebrate the summer solstice.

Image of the 125 Anniversary logo Summer officially begins this week as the summer solstice marks the longest day of the year. For Americans, summer will begin either on Thursday or Friday—depending on which time zone you live in.