If holiday travelers on the same plane compare what they paid to fly, they're likely to find quite a spread, depending on when they bought their tickets.

Fares for travel around the holidays have been rising since late summer. Christmas fares are now running 4 percent below a year ago, and the gap is likely to disappear soon.

Contrast that with a year ago, when the airlines essentially put the holiday travel season on sale. With the recession in full force, airlines used discounting to fill seats. People who waited to book holiday fares last year saved money. This year, holding off could cost you.

Most carriers pushed through a $10 fare increase last month. For the holidays, big airlines added a $20 surcharge each way on popular travel days closest to Christmas and New Year's.

Tom Parsons of BestFares.com compared holiday fares purchased on July 1 with the same itinerary booked on Nov 2. Several had risen 50 percent or more.

Airlines have been shrinking to match a decrease in travel. With the supply of seats more in line with demand, carriers have been able to raise fares close to where they were last holiday season.

Average Thanksgiving fares are up 2 percent to $351, according to Bing Travel, the fare-watching website owned by Microsoft. The average Christmas fare is about $370 roundtrip, slightly below a year ago.

Thanksgiving fares "are up quite a bit even from where they were at the beginning of October," said Joel Grus, who tracks fares for Bing Travel. He also thinks Christmas fares will soon be at last year's levels.

Given the upward trend in fares, Grus advises travelers to book now. Check fares several times a day. Sometimes seats become available at a lower price.

Of course, the cheapest ticket is purchased with frequent flier miles. Airlines make only some seats on each flight available for purchase with frequent flier miles. Some are still available, but Randy Petersen, editor of InsideFlyer magazine, said they were getting scarce because most holiday travelers started booking those as early as August.

Some other things to consider this travel season:

— Some fees are higher. In the last year, airlines began charging $5 more each way if you pay your baggage fee at the airport instead of online. Most airlines now charge $15 to $20 to check your first bag and $25 to $30 for the second.

Southwest still allows two bags for free. JetBlue Airways allows one. Virgin America added a fee of $20 per bag this fall.

— Consider a change in plans. If you want your own personal airfare sale, pay attention to which days you fly. A popular Thanksgiving itinerary is to fly on Wednesday before the holiday and return on Sunday. Grus said fares run about 25 percent cheaper if you fly on Tuesday and return Saturday. Flying on Thanksgiving Day and returning on Monday can save fliers about 30 percent on average, he said.