On May 20, the Sun is always in Taurus or sometimes it could be in Gemini?

SOME YEARS Sun leaves Taurus and enters Gemini SOMETIME during May 20th.

Other years, it makes this change sometime during May 21st.

If someone is born on either May 20th OR May 21st, the ONLY way to know which Sign Sun was in is to do up their personal birthchart. Which you should always be doing anyway, since Sun is only 2% of the entire birthchart.

So that requires knowing (1) the day-month-year of birth, (2) the EXACT minute & hour of birth, and (3) the place of birth. The net abounds with websites that do up birthcharts for free. I recommend alabe.com or cafeastrology.com.

Some scammers try to tell you that a May 20/21 birthday is "on the cusp". But that is just a gimmick to not lose readers. The Sun is either in Taurus or it is in Gemini, and it is not in two places at the same time. OTHER planets might be in the other Sign giving some of those characteristics, but it is not because Sun is "on the cusp". You can always tell who understands astrology because

- they always say the person has SUN in Taurus ... not that they are a Taurus

- they NEVER say someone is "on the cusp"

You see, where Sun appears to be depends on where Earth is in her orbit ... we look at Sun and there is a particular Sign (NOT constellation) behind the Sun.

The reason the dates vary are because our calendar is shorter than Earth's orbit around Sun. That is why we have Leap Year every 4 years, to re-calibrate our calendar to match Earth's orbit.

always in Taurus.

It's probably possible in some parts of the world in certain year for a few hours, say in the UK where the dates change first. I'm not going to look it up. Most likely, though, in the US, where the east coast is five hours behind the UK, it will be in Taurus. If you're wondering about a specific case, then post the birth time, year, AND PLACE to get a specific answer. I'll do it if I see it.