I also live in the US, and I'm somewhat amazed that anyone here might not be familiar at least with the song. I have known it and the history you are commemorating at least since my teens, but that was admittedly a long time ago. A version of the song was popularized in this country by a recording in the 1950's or early 60's that was played a lot on the top-40's radio stations. In fact, there was a movie about Gallipolli starring Mel Gibson, but that was in 1981, when some of our members were children or not yet born.
It's good to know that the date is still honored in Australia and NZ. I'm reminded of the famous quote by George Santayana: "Those who cannot remember the past are condemned to repeat it."