In the hidden-object genre of casual games, there's a substantial and largely regrettable subgenre of romance-centred stories, within which are a few games about planning weddings. In certain respects, D-Day plays very much like Dream Day Wedding and its ilk. The gameplay mimics hidden-object in that there's effectively nothing to do except to find all the things and put them where they need to be. To-do lists and now-do-this instructions strive to preserve the player from the slightest confusion. The world is sparkly and conflict-free, and the bride is kept safely generic; her intended is absent, barely mentioned at all, and seems irrelevant to the whole undertaking. (It's probably unfair to consider D-Day as literature rather than a straightforward iteration of a standard fantasy; it's certainly not attempting anything more than the latter, which means that I'd be unlikely to like it much regardless of its other qualities.)
The two key elements of casual games that it lacks are a high level of aesthetic polish (in IF that might translate to lavish prose and meticulous scenery implementation) and an intuitive, idiot-proof, silky-smooth user interface. Both of these could have been substantially improved by testing.
It's probably fairest to think of D-Day as a My Apartment game. In that light, it's quite respectable: it demonstrates some thought about structure, it provides immediate objectives, it's trying to be considerate of the player, it can be won without ever getting stuck, it doesn't have the cynically lazy my-game-sucks attitude that typifies My Apartment.