Today is the Feast Day of St. Thomas the Apostle. He is best known as Doubting Thomas because he did not believe the reports of the Risen Christ. Thomas wanted proof. In a way, he represents the rest of us in the desire to have hard facts about the Resurrection. When Thomas puts his finger in Jesus's side, he becomes the witness for all of us. The doubter is convinced, as Thomas exclaims, "My Lord and my God". This encounter is in today's Mass Gospel reading (John 20:24-29).
The USCCB has a great site that gives the readings for Mass everyday. Click on http://www.usccb.org/nab/today.shtml to see today's reading.