Every 27.3 days the Moon comes back to the exact degree it occupied when you were born, and a chart cast for that moment is read as a forecast for the month.

Solar returns — birthdays — get one chart a year. Lunar returns get thirteen, which makes them the highest-resolution predictive tool in common use and, for the same reason, the one most likely to find a pattern in noise. Practitioners cast them for the place you are actually in, not where you were born.