Table schema:
CREATE TABLE table_11900773_6 (first_day_cover_cancellation VARCHAR, date_of_issue VARCHAR, theme VARCHAR)

Question: Where is the first day cover cancellation for the 3 April 2008 IIHF World Championships, Quebec City stamp?
SQL: SELECT first_day_cover_cancellation FROM table_11900773_6 WHERE date_of_issue = "3 April 2008" AND theme = "IIHF World Championships, Quebec City"