The episode starts on Monday 17th December 1973, a week before it first aired: Albert tells the Christmas tree fairy it's "Your first Common Market Christmas," the UK having joined the European Economic Community (EEC) - AKA the Common Market - on 1st January 1973.
Harold was 6 months old in 1932 when Albert gave him the Christmas tree fairy, 41 years ago. His mother died two days before Christmas 1936.
Albert had to turn in his ARP helmet in 1940 after crashing into the church with a haul of roof-lead whilst firewatching.