TODAY'S Our President's Birthday . . . All Fares Reduced 50 Percent!"

"Smile, It's Tuesday . . . Go to Vegas for 30 Percent Off!"

"Try Our Surprise Flights - $50 off if you buy and fly between noon and 4 this afternoon!"

If you dipped into the future, is that what you'd see in the way of U.S. scheduled-airline offers?

Certainly not! Every right-thinking company wants to unclutter, to get rid of its multifare lists and settle down to selling seats, not explaining rules. Furthermore, the Civil Aeronautics Board is politely pressing for plain, everyday lower fares.

But . . . well . . . hmmm . . . without the cutesy come-ons, precisely these kinds of "specials" continue to shoot out, and it's probable that the new "floating fare" system just served up by the CAB will lead to an experimentation season that snaps, crackles and pops with more.

Permission to float domestic fares (that is, to raise and lower rates, within limits, on routes inside the 48 contiguous states without first getting approval) was immediately followed by a number of fare increase for Friday and Saturday flights on the part of TWA and a reduction of all its first-class fares by Delta. Additional ups and downs are indicated by a large number of new route changes in the offing.

When you put it all together, does it mean that identical fares are out and "every day in every way a different fare" is in? Does looking for the lowest price mean having to gear up for some serious shopping?

Those are good questions, and it's just too bad that there are no equally good answers. At present, only one thing is perfectly clear: If money matters, you'd better stay tuned, and meanwhile tuck away a few nuggets on what's known and what isn't. For instance:

Q: How often are domestic fares likely to change?

A. Other than general rate increases, which usually blossom out in May and November, specific forecasts are few and far between. Those two increases are worth remembering, though, since in most cases it's possible to buy a ticket ahead of time at the lower rate even if you want to fly when the higher rate is in effect.

For the most part, industry executives seem to feel that all the major fare reductions that can be made have been made. At the same time, lots of companies are now applying for and getting new routes - and tending to offer attractive introductory rates to get people to fly them.

Moreover, the CAB hopes and expects that airlines will dream up discounts that are quite different from those now offered. The crystal-ball gazers see the fading-out of fares based on how many days to stay away or how many days ahead you pay and new, more cost-related fares creeping in - perhaps a return to different prices for flying on different models of planes, most likely more "discounts" to stimulate travel during slow travel periods (If it's Tuesday, it's $54.50."), maybe just a single system of first-class, coach and no-frills fares on the East Coast north-south routes and another group for cross-country East-West.

There's also the possibility that if the CAB rewrites more rules, it could spark more types of changes. Now, for example, at crowded popular airports like New York's Kennedy, the airlines meet and divvy up the limited number of landing and takeoff slots available. If instead they had to bid for the places, speculates one official, might it not lead to "more competitive ticket prices"?

Q. If lots of changes do come, how can a buyer cope?

A. Even if "lots" turns out to be only a few that affect you, you might best begin by changing your shopping habits to include the key question, "Is there anything coming up in the way of fare changes?"

Airlines still have to file almost all tariff changes (meaning changes in prices and fare restrictions) at least 45 days before putting them into effect. If, in talking to airline representatives and travel agents, you do learn of something good just down the road, you can then at least consider shifting your plans to take advantage of it.

Bear in mind, though, that not all of the fares that are announced - reductions or increases - actually come into being. TWA's intended fare increases in local markets (such as New York-Minneapolis, Boston-Tucson) were not matched by competitors, so TWA dropped them. But Delta's plan to reduce first-class fares across the board was quickly followed by similar announcements from other airlines, so those changes will come to pass.

Q. How far ahead do you need to start shopping?

A. At present, the biggest discounts on the books go with tickets that you have to buy at least 30 days ahead, usually called Super Savers. But especially for peak-period travel at a discount, you need to start even earlier, because such cheapies can be new in number and they're sold first-come, first-served.

Q. How do you know when you've hit the bottom price?

A. In the end, you just have to take someone's word. Since the lowest fares generally come with one or more restrictions (on the length of stay, the day and time at which you fly, how far ahead you buy), and range from roughly 15 percent to 50 percent off the standard coach fare, you can easily arrive at conclusions about simple point-to-point fares - that is, on flights from major city to major city.

Nonetheless, occasionally you can find a roundabout routing with a combination of discount fares that will undercut the cheapest "simple" ticket or (much more likely) undercut what you'd pay when going between smaller points where discounts are a rarity.

It's of course up to you to calculate if you want to spend the shopping time required, considering the possibly small saving that might be involved. It's likely to be onerous, time-consuming work, which is one reason more than a few travel agents are no longer interested in helping you do it.

Q. If you can't get or fit into a Super Saver, where should you look if you want to build your own cheapie?

A. Right now, stay alert for "introductory" offers. Also, night flights and hop-skip-and-jump service, i.e., indirect and one or two-stop routings using a single airline.

Q. Do you have to shop all the airlines?

A. It's the safest way for little spenders, although most fares offered by one airline are matched by its competitors. But floating fares and new "let-there-be-competition" attitude at the CAB may result in some time lags in which one airline, advertising heavily, will have a few days or weeks with fares that haven't yet been cloned by the friendly opposition. On heavily traveled routes, such advantages are not likely to last for long.

Of course, cheap-seat hunters shouldn't overlook asking travel agents about charters. There's only a skimpy selection within the United States (mainly to Hawaii, Florida and Las Vegas), and more get announced than fly, yet many customers come away eminently dissatisfied (though there are often negative factors to consider, such as long delays in departures and cancellations that can botch up a vacation).

Q. What about shopping for foreign flights?

A. It's pretty much the same story, except that last-minute flyers stand a good chance because of bargain "no reservations" flights to Europe (Skytrain, standby fare).

Latest cheap offers on reserved seats on scheduled airline flights include a batch of lower fares on Icelandic to Luxembourg, SAS to Copenhagen, and National to Amsterdam. Starting probably in March and April, there'll also be special low-priced reserved-seat services to Brussels, Amsterdam, Guam and Hong Kong, with tickets for sale mainly through travel agents. Capital will be the carrier to Brussels, World to the other places.

For the most part, however, you need to shop well ahead for international tickets, too, since the majority of carriers offer their biggest discounts on fares that are sold in limited quantities and at least 21 days ahead.