NEW YORK ( MainStreet) — Pick your preference. You could fly nonstop from San Francisco to Rome for $1,125. Or you could pay $1,650. Same carrier. Same seat.

The difference? The sharply cheaper flight is now, while the pricey ducat is is the height of summer when of course everyone wants to go on holiday.

It's not just Rome. Just about any destination can be had at a sharp discount by knowing when the crowds head there and avoiding the stampede.

Oh, incidentally, Rome weather now is ideal for a vacation that involves lots of walking which is what museums and old ruins involve- highs in the mid 40s, dry. In summer, that temperature can double, so can the humidity, and the crowds can be counted on to triple, meaning that zero wait to get into the Vatican Museum today suddenly involves standing a few hours in a sweaty, slow moving line.

The takeaway: timing is everything in smart travel and the smartest travelers invariably go against the flow of the crowds. Fewer travelers mean lower prices, shorter lines, and a lot better holiday.

You want to know the exception to this frugal travel rule? There is a big one. Off-season travel works a lot less well to destinations that involve sun and (pleasant) heat. Call it the Bathing Suit clause. If you are intent on bringing one, forget the off-season. Yes, you can score huge discounts on hotel rooms in Nantucket right now. The charming Seven Sea Street Inn, for instance, has availability at $125. That same room in July fetches $375 and probably it will sell out.

But what would you do in Nantucket in the dead of winter besides talk about Moby Dick ? And know that there may not be another soul there to talk about Melville with.

If a bathing suit plays a central role in your vacation wardrobe, probably, you are stuck paying full freight for peak season travel.

For the rest of us - who haunt museums, clothing stores, flea markets, enotecas, restaurants and the like - the off-season discount is ours to grab, because in most every respect the weather is irrelevant.

Another downside to off-season travel worth noting: hours of daylight get very short, especially in northern Europe in winter. In Berlin now there are under eight hours of daylight daily, and a lot of that is murky gloom. In July, the day stretches a glorious 18 hours of sun that often is bright. But when you are sitting in a kneipe nursing a lager by a radiator, how does it matter if it's dark and frigid outside? It does not, and Queen Nefertiti - Berlin's art jewel - receives visitors year-round at the Neues Museum.