When is the sun directly overhead?

In another thread, aussie said:

...The tropical sun [on St. Croix] is straight overhead twice a year.

I've been wondering how to figure out on which days the sun is most directly overhead at a given latitude.
At 23.5 N (Tropic of Cancer), it's on our summer solstice (~June 21), which is once per year.
At 23.5 S (Tropic of Capricorn), it's on our winter solstice (~Dec 21), which is once per year.
Everywhere else in the tropics, it's twice per year.
At 0 N (Equator), it's on the equinoxes (~Mar 21 and ~Sep 21).

But when is the sun is most directly overhead at a specific latitude like 18 N?

I just found the chart below at http://www.physicalgeography.net/fundamentals/6h.html

For 18 N (the approx. latitude of USVI), the chart shows the sun is directly overhead around May 5 and Aug 4.

Elsewhere on that website, I see that “Subsolar Point” is the term for the location on the Earth where the Sun is directly overhead.