Semi formal for women is the same as cocktail dress wear. There are lots of sites explaining dress codes if you search "define semi-formal wedding attire" such as "the knot.com" or "everafterguide.net". And they provide other tips as well such as lighter colours for daytime than evening weddings, better to over dress than to be too casual, and never wear white to a wedding..... So, your long dress would work great, lilyapril, and if you wish to wear a short dress, that's probably what most women will be wearing.