Those born on May 21 can be of the sign Taurus or the sign Gemini.
As you already know from this website, this is because the transit of the Sun through the Gemini sign takes place that day at a certain time, therefore, depending on the time you were born, the Sun is still in Taurus or already in Gemini.
Taurus is stability, standing, firmness, and the need to touch and have. Gemini is extroversion, communicating, talking, and needing to have something to do or someone to chat with. Both are very different, so knowing what sign you are will be very important.
If you were born on May 21, you can ask us in the comments what your sign is by entering your day, time and place of birth. Only with these 2 data can we really know where your Sun is in the Natal Chart.