Yellow sapphire is best worn on a day on which Jupiter’s constellation (nakshtra) is ruling. This happens when Moon is travelling through that Nakshatra. Daily panchang in newspapers shows it every day.
Following are the three Nakshatras of Jupiter:
Poorvabhadrapad (Pu Bha)
The moon should be waxing (Increasing), moving from Amavasya (new moon) to Full Moon.
These two conditions should be met.
Wishing you good luck, God bless you and, above all.