In the Roman Empire, which calendar placed the summer solstice on 24 June?

In the Roman Empire, the Julian calendar placed the summer solstice on 24 June.

The Julian calendar was introduced by Julius Caesar in 46 BCE and took effect in 45 BCE. It used a 365-day year with a leap day added every fourth year, a reform designed to bring the civil calendar back into closer alignment with the solar year. In the Roman world, the summer solstice was therefore associated with 24 June under that calendar’s reckoning.

The date was not an unchanging astronomical truth. The Julian calendar’s year was slightly too long, so the calendar gradually drifted relative to the seasons. By the time of the First Council of Nicaea in 325 CE, the spring equinox was conventionally set at 21 March for Easter calculations, and the solstice’s calendar date had shifted earlier than the traditional Roman 24 June.

The Gregorian calendar, introduced in 1582 in Catholic Europe, corrected much of this drift by revising the leap-year rule. A common mistake is to treat 24 June as the modern astronomical date everywhere; it is better understood as the Julian-calendar date associated with the Roman Empire’s midsummer reckoning.