This answer piggy-backs on the comments under Nathan Tuggy's answer.
Everyone knows there could be possible negative consequences for voting poorly on Election Day; however, D-Day would be the wrong analogy to use. Assuming the elections are largely peaceful and uneventful, the worst a voter could encounter on Election Day is a long line.
That said, the analogy might be fitting if one poll worker was talking to another. That worker might say, "The elections are coming soon. We will be ready to set up on D-Day." Considering how much equipment needs to be set up and ready to go at such an early hour, that analogy would be entirely fitting and readily understood in context.
I also don't agree that this is "only a valid usage in British English." As an American, I've seen it used from time to time, although it can be difficult to find examples because so many of the hits will be related to the World War II invasion. However, here is one example, taken from a 2015 Florida business news article:
Both companies [Uber and Lyft] are fighting the taxi medallion-owning establishment, cabbies protesting in France, a backlash from New York's Mayor DeBlasio, and now half of South Florida. Broward County is pushing them out, a driver was arrested in Key West for driving an 'unlicensed vehicle for hire,' where there isn't even Uber service yet, and now as they plan their next moves toward full legalization, Uber is ensconcing itself down here pretty much permanently, ready for D-Day.
And from CIO, in an article about promotic events:
Once you’ve planned an event and decided on the dates and the venue, you need to market it. There are conventional methods for building hype and awareness around events. Use of social media is one of them. Building a steady tempo on social media as D Day approaches is a strategy most commonly used by many brands to create awareness.
Moreover, the American Heritage Dictionary defines D-Day generically as:
- The unnamed day on which an operation or offensive is to be launched.
That said, having scoured through some news articles, it does seem more commonly-used in BrE publications. But that wouldn't "invalidate" usage on American soil.
One other footnote: We don't typically use the definite article with D-Day. So, when preparing for a wedding or other big event, you would use it like this:
Imagine you have a house party. As D-day approaches, here's what you're ticking off your mental checklist.
Imagine you have a house party. As the D-day approaches, here's what you're ticking off your mental checklist.