MOG dress

Consider asking your seamstress stitch the back closed. If the slit bothers you there may be a reason worthy to explore.

I've had another thought:-
*if the facings/fold of material are quite wide on the reverse side of the split then sew on two flat buttons about an inch from each edge(out of sight when wearing anyway).
*spread the split open just to the point of gaping.
*decide how high you are happy to show a bit of back leg.
*cut to length a wide..ish piece of elastic to reach slightly beyond each button and across the split when spread.
*cut two slits at each end of elastic for button holes and button elastic across split.
I'm thinking the elastic will prevent split from gaping further than wanted but help with movement (can be unbuttoned if needs be for loo purposes!)

H.mm probably decide how high you are happy to show a bit of back leg before sewing on buttons!

Lilylavender your outfit sounds wonderful. Wear it and enjoy the day. Also any photographs will only (normally) show the front, so don't worry about the split. I will also be a MOG in a couple of months and have bought a sleeveless dress as it will be very hot where we live. I have also bought a silk shawl to wear to cover the upper arm area as my upper arms aren't what they used to be!! (actually the rest of me isn't what it used to be either.) Having had three daughters marry I think this will be the most relaxed, as I have learnt that the eyes will be on the bride and groom not the guests. Have a fabulous time.

I think the last few posters were a bit late to the party ... the wedding was on Sat 5th Oct, as stated in the OP

Hope you had a wonderful day Lilylavender!