is 前日 just yesterday or what

ok wait im losing my mind over 前日 like i know 前 is before but is it actually different from 昨日?? or is it 先日? wait is this a dumb question lol my brain is not 柔軟 enough for this. i keep reading it as zenjitsu but then my brain reacts with mainichi because of 毎日 and i just freeze. are they all the same or am i just making up rules at this point