Bingo! Since World War II, Celtic haven't won a national cup final played in the same year as a Royal wedding.
Full details:
1947 – Wedding of Princess Elizabeth & Prince Philip. One Scottish Cup Final and two League Cup finals, did not play in any.
1973 – Wedding of Princess Anne and Captain Mark Phillips, Lost Scottish Cup Final (Rangers) and League Cup Final (Dundee).
1981 – Wedding of Prince Charles and Lady Diana Spencer. Did not play in either final.
1986 – Wedding of Prince Andrew and Sarah Ferguson. Did not play in Scottish Cup Final, lost League Cup Final (Rangers).
1999 – Wedding of Prince Edward and Sophie Rhys-Jones. Lost Scottish Cup Final (Rangers), no League Cup Final played in 1999.