"The day marks the estimated calendar date when humanity’s demand on the planet’s ecological services (which produce renewable resources and assimilate wastes) outstrips what the Earth can supply. This means that for the rest of the year, we are taking more than is regenerated, operating in Overshoot. Last year, Earth Overshoot Day was August 19th. We first went into Overshoot in the late 1970s, and since then the day has crept ever earlier on the calendar. This means we are using the ecological resources of just over 1.5 Earths."
Note: This is a conservative and optimistic assessment. The "calendaring" suggests a simple, linear process that could be reversed; the actual effects of overshoot are more likely to be hyperbolic and irreversible.