Rebecca Black's Friday is thought of as just some girl talking about her life, and the fact that she likes Fridays.
But in fact it is a far darker and introspective commentary on her apparent midlife crisis. For her, she has reached the middle of her life (which happens to be Friday), and she has come to a turning point. She can either take up the front seat and take charge in her life, or simply sit in the back seat and go where life takes her.
In a somewhat dark statement, she claims that everyone is looking forward to old age, and even death (the weekend). She realizes that what she wants in her life at the moment it to live to the fullest, and have fun fun fun fun. So she hops in her car and contemplates fun, again facing her decision to take charge or go with the flow. She ends up deciding to embrace her current situation.
But in that embrace, she thinks about her past (Thursday). She doesn’t like her past, so she gets herself to appreciate the present. But then she looks again to the future, Saturday, then Sunday, and life (the weekend) will be over. This thought depresses her, and urges her forward in the celebration of the present.
She forgoes her decision on whether to take charge or not, and decides that the question itself is flawed. It is in fact possible to do both, and still live a full life. The song ends as she repeats her excitement in her new found appreciation for the present, and acceptance that the future is inevitable.