How to calculate the true length of day on a celestial body? When I only know the sidereal rotation period, how do I determine the true length of day on a body? 

We know the Earth's sidereal day doesn't have full 24 hrs unlike the true length of day. It's similar on Mars where the sidereal day isn't as long as a Sol (about 24 hrs 40 min) either. So how do we calculate the true length of day on a planet?

Let's do it on the example of the planetoid Ceres: its sidereal rotation period is 9 hrs 4 min and 27 sec. How do we calculate the true length of day on Ceres?
If you know the length of a sideral day and the length of a year, you can determine the length of a solar day. 

From Wikipedia:



After one year, there will have been one more solar day than sideral day. Therefore :

solar day = sideral day / (1 - (sideral day/orbital period))