Why can't the wedding between Juliet and Paris take place a day earlier?

Apparently it could. It's not that hard if you don't care whether you have any guests and you have a staff of servants to make a big meal on a moment's notice. All you need is an obliging officiant and a bridegroom who is in the loop.

Capulet does exactly this. He decides that the wedding scheduled for Thursday should take place Wednesday morning. He makes this decision late on Tuesday.