In Act I, Scene 2, a servant has been assigned to deliver invitations to a banquet at the Capulets' home, but the servant is almost totally illiterate. Just by chance he asks Romeo for help in reading the list of all the names of the people to whom he is supposed to deliver the invitations. Romeo accommodates the man and naturally asks him where this elaborate banquet is being held. The servant replies:
Now I'll tell you without asking. My master is the
great rich Capulet; and if you be not of the house of Montagues,
I pray, come and crush a cup of wine. Rest
Romeo's friend Benvolio encourages Romeo to crash the Capulet's party. This should be easy enough to do, since it is such a big affair and, according to the servant, is practically open house to anyone who is not a Montague. Benvolio is trying to help Romeo get over his love-sickness for Rosaline because Romeo is no fun to be with in his solitary, melancholy aloofness. Benvolio tells Romeo:
At this same ancient feast of Capulet's
Sups the fair Rosaline whom thou so lov'st;
With all the admired beauties of Verona.
Go thither, and with unattainted eye
Compare her face with some that I shall show,
And I will make thee think thy swan a crow.
Romeo goes there hoping to see Rosaline, but falls in love at first sight with Juliet.
Coincidentally, Juliet's father has encouraged Paris to come to the party in order to be able to see all the beautiful girls of Verona and compare them against Juliet. Capulet thinks his daughter is too young to marry and is hoping that Paris, who has formally asked for her hand, will meet a girl who appeals to him more than Juliet.
Capulet sends a messenger around town to invite guests to the party; unfortunately, the messenger can't read. Romeo and his friends encounter the messenger in the street, and Romeo helps him out by reading out the names on the list. That's how Romeo discovers that his (unrequited) love interest, Rosaline, is invited to the feast. He goes to the feast to be near her.