day of abstinence; day of purification
A day set aside for religious purification, often involving dietary restrictions such as avoiding meat, in Buddhist practice.
I make it a rule not to eat meat or fish on days of abstinence.
精進料理 is the vegetarian cuisine associated with Buddhist practice, often eaten on 精進日.
From 精進 (shōjin, 'devotion, purification') + 日 (bi, 'day'). The reading び is a rendaku form of ひ.