I'm going to be what Angie calls "a mare" about this and insist that when your life doesn't call for dressy very often, this is when it's really important to keep that outfit in reserve.
If you dress up all the time, the likelihood that you will panic when a dressy occasion arises is low. It's those of us who live in jeans and boots who will not have a go-to source for dressy clothes, who will not own the right evening shoes, who will not own an evening bag, AND who will not be likely to own a dressy coat to go over your dressy outfit if your event falls in the dead of winter. (Ever notice how many dressy dresses are sleeveless? Not great when it's January in New York. And you're 53, with 53-year old arms.)
Janet, all your points are spot on, so I'm suggesting that it's important to keep the dressy outfit ready-to-go. Try it on, get it tailored if need be. Make sure you have one for warm weather and one for cold, if applicable. Get the right shoes and the right bra.
This is a fire drill, ladies. Don't wait until that heavy envelope shows up in your mailbox. Sure, you'll probably get a wedding invitation two months ahead of time. But there is no guarantee that in those two months, anything seasonally appropriate will be in the stores.