The Birthday Express will be the first even though it has not yet been released on DVD up until May 17th 2011 and contains two Season Twelve and Thirteen episodes respectively.
Afterwards, the 2009 CGI debut special Hero of the Rails will be broadcast.
The description for the TV event reads; It's Thomas' birthday, and we are celebrating on Sprout!
Catch the Thomas specials Birthday Express and Hero of the Rails on Saturday and Sunday, May 14th & 15th at 1pm ET and an encore of Birthday Express each day at 5pm ET.
Thomas' Birthday Bash Marathon broadcasts on PBS Sprouts starting from Saturday May 14th at 1pm ET.
Thanks to MrTrainfan1000 for the alert.