3 of 12 · May 21 – June 21 · Air · Mutable
The twins hold two points of view at once, which is the whole idea of the sign: exchange, translation, and the suspicion that any statement has an opposite. Gemini rules the lungs and the hands — the organs of breath and of gesture.
The dates above are the conventional ones and hold for most years. The Sun actually crosses into Gemini at a slightly different moment each year — by up to a day — so a birth on May 21 or the day before can land on either side depending on the year and the hour.
If the birthday is on that edge, check it with the zodiac lookup using the exact time and place rather than a printed range.