That all the world will be in love with night
And pay no worship to the garish sun.
O, I have bought the mansion of a love
But not possessed it, and, though I am sold,
Not yet enjoyed. So tedious is this day
As is the night before some festival
To an impatient child that hath new robes
And may not wear them.
At this point in the play, Romeo and Juliet have been married, but have not consummated their marriage. They found time to get secretly married, but not secretly have a wedding night . . . yet. That will happen soon. Juliet is thinking it will be quite soon, and can't wait. She longs to see Romeo again. How much does she long? Well she's practically in agony about it.
She compares her longing to someone that has just bought the house of their dreams but hasn't been allowed to move in yet. She calls the day "tedious." But not just any tedious, as tedious as the day is before some awesome festival party. I would liken it to a high school girl attempting to "patiently" wait for her high school prom to start. She then admits that she's like an "impatient child" that can't wait to wear his/her new outfit.
All in all, Shakespeare does a nice job of describing her agony of expectation because he has Juliet compare it to things that people still are very familiar with.