Most notably, it has been written on (sewed onto) every challah cover I've ever seen. For example:
So what's it all about?
Meaning & Usage
It means "in honor of/for the honor of Shabbat and holiday." As a grammar nerd, it's interesting to me that yom tov isn't written yomim tovim, the plural. However, each time Shabbat and yom tov fall on the same day (happens pretty often), we make the bracha "has commanded us to light the candles of Shabbat and yom tov." That bracha phraseology may be used at other times during the chag, but none come to mind for me right now. So I think the theory is that the cover is useful for any specific day you might use it, rather than the general idea of Shabbat and holidays throughout the year.
You may often see it shortened to just "Shabbat v'yom tov," as you see on 2 of the 3 challah covers pictures. (Those are 3 of the 4 I own. Why do I need 4? I have no idea, but there was a wedding involved.)
Does it have to be both Shabbat and yom tov to use an item that says L'kvod Shabbat v'yom tov? Nope, it's just a catchall for all the uses you might have. Use it on Shabbat. Use it on yom tov. Use it when they fall together. But don't use it on Pesach. Unless you set one aside for Pesach (hey, that's a great idea for one of those 4...).
How Else Is It Used?
Nearly anything can be done lekavod Shabbat v'yom tov. In fact, the most common place you might hear it spoken is in the grocery store. There are many famous stories about rabbis or other pious people who saved the very best food for Shabbat, even in the poorest of conditions. That idea continues today, and you may overhear people debating whether to buy a nicer version of some food or a new food lakavod Shabbat. This can be done any day of the week, so long as it is set aside for the coming Shabbat.
I would guess that haircuts and buying flowers come in second and third, but it's a dead heat for which one should take which place. If you're going to buy flowers or get a haircut, it's best to do so late in the week (Thursday at the earliest but preferably Friday is what some people say) in order than it can also be in honor of Shabbat, even if you intended to do it anyway. This is particularly relevant to haircuts.
You should also try to save newly-purchased clothes so you can wear them for the first time on Shabbat, in honor of it. It's hard to wait sometimes (so guilty here), but it's a very common custom and a beautiful idea.
It can really be anything. I know of a working mom who doesn't do much cooking during the week, but always cooks for Shabbat to honor it. Her homecooking is so unusual that she's been able to turn it into something that can bring extra honor to Shabbat and joy to her family.
Do you (or others you know) do anything else l'kavod Shabbat or yom tov?
But How Do You Say It??
The better question is how one pronounces it. I have found three pronunciations (so far):
L'KaVOD: Luh or Le being the best approximations here for L'
LaKAvud (like "mud")
Do you know a pronunciation I missed? Which one do you use? Where have you seen it written besides on challah covers?
And what special thing will you do to honor Shabbat this week?