For me, "lace pants" would be too cold for true winter temps, though March can go either way around here. (Sometimes serious snowstorms, sometimes balmy spring temps. Impossible to predict.) As long as you'll be indoors the whole time, you could always stash tights underneath if it was really, really cold -- just don't count on going too exposed with your footwear.
Do you have or could you acquire or borrow a sparkly jacket to go over the lace pants? With a plain layer beneath (which you could use to make things temperature-adjustable -- tank if it's warmer than usual, or if it's cold, a deep black layering tee in a dressy fabric, like the one below), you could be sure that you were fancy enough but also comfortable if you end up facing unusual weather in either direction.
One caveat, kerry: resist the temptation to wait until the wedding is approaching to decide which layering piece to buy. Buy one of each and do it now. If you wait until it's clear what the weather will be, the piece you need will suddenly be impossible to find.
I'm posting these ones from White House Black Market because the fabric is all synthetic, so it will not "gray out" on you. No cotton. These pieces are dressy-looking and will stay a deep, true black. Just don't go too snug in the fit. They don't run small, but they are meant as layers, so by design they don't have an overly generous fit.
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