1919 The British Admiralty recommended that helium, a non-flammable, lighter-than-air gas, was a safe substitute for hydrogen filled balloons and airships
1937 The airship Hindenburg, the largest dirigible ever built, burst into flames upon touching its mooring mast in Lakehurst, New Jersey.

It's odd that those two things happened on the same date. I wonder why they hadn't switched to helium for the Hindenburg?