This can be proved from a number of sources and is covered in our literature. Notice the following sources:
1) Our booklet, "Should Christians Observe Easter or the Passover" discusses this subject. It is located here:
http://www.garnertedarmstrong.org/pubs/passover.htm
2) Companion Bible---Appendices 156, 157, and 158
156--- http://www.intercontinentalcog.org/Appendix/Appendix_156.php
157--- http://www.intercontinentalcog.org/Appendix/Appendix_157.php
158--- http://www.intercontinentalcog.org/Appendix/Appendix_158.php
3) An article on the Internet, "Good Friday is a Myth, Jesus Died on a Wednesday!!"
Located here: http://ad2004.com/prophecytruths/Articles/Prophecy/3days3nights.html