I have no idea what semi formal means.

I have been invited to a wedding at the end of June, and the invite says Semi-formal. Googling it is just kinda confusing, different sites say different things. Do you think this would work? Is it too short for semi-formal? The family is super catholic, so I am a bit concerned about the bare shoulders, but the wedding is a military base wedding and im not sure if church clothing expectations would be the same. I personally rather dislike dresses to begin with, but am sucking it up fo this event.