Paris is the man that Juliet was set to marry. There are some debates that are ongoing with regards to when Juliet was supposed to get married to Paris. In the beginning, the wedding was set to happen on Wednesday . But it was decided that this was too soon so it was moved to Thursday.
No matter what day the marriage would have occurred it might not be valid anyway because, at that time, Juliet was already married to Romeo. This means that the second marriage will only be considered null. Take note that Juliet did not want to follow her father so she was threatened that she will be disowned if she did not obey.
Technically, it wasn't supposed to happen. She was already married, and at the time it is set in, your spouse has to die before remarriage, and Romeo is still alive.
Technically, this question has two answers. The arranged marriage was originally supposed to take place on Thursday, but after being overjoyed Lord Capulet changes the date of the wedding to Wednesday.