I think I see it as 1937, and the date as 5 SP (September). Assuming I'm right with the date, there are a couple of reasons why 1937 fits better:
(a) 5 Sep 1937 was a Sunday, so it makes sense for a postcard to be sent cancelling a visit on the Tuesday. 5 Sep 1939 was a Tuesday, so any reference to a Tuesday would be for the 12th, and I'd expect to see "next Tuesday". (Sunday collections - don't you just miss 'em!)
(b) Britain declared war on 3 Sep 1939, so I think it's rather unlikely that someone would be planning a week's trip a couple of days later; or if they were, that they wouldn't mention the war at all.