Twenty-eight

A day on the Moon is 28 Earth days. Makes sense!

http://www.madsci.org/posts/archives/mar97/849679535.As.r.html

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Got interested in the notion of the sidereal ie with respect to
the fixed stars (as opposed to the planets, which move around).

In Latin, the word for a star or constellation is sidus. Latin speakers used that word to form desiderare ("from a heavenly body") and considerare ("to think about a heavenly body"), which were adopted into English as desire and consider. Sidereal, another sidus creation, was first documented in English in 1642. Thirty-four years later, an astronomer coined the phrase "sidereal year" for the time in which the earth completes one revolution in its orbit around the sun, measured with respect to the fixed stars.
Merriam-Webster

Because the sun itself is moving, the sidereal year is some 20 minutes longer than
the tropical year (from one equinox to the next).

When using the term sideral month with respect to the Moon, the sideral month
is shorter than the synodic (gr synodos, conjunction of two bodies). This is because
the earth is moving around the sun, and the moon needs to catch up.

The sidereal month is 27 days, 7 hours, 43 minutes.

The synodic month is 29.5 days.