The Holy Door or 'Porta Sancta' is only open during a Holy Year (Jubilee), which occur every 25 years (the last one in 2000).

On the first day of a holy year, the Pope strikes the brick wall with a silver hammer and opens it to the pilgrims.

The message imparted by the Holy Door is that God's mercy reaches out to mankind's frailty.

From inside the basilica, you'll notice that the door is walled up.