So what can we expect in their finales? And how will they measure up alongside some of the celebrated series' finales in history.
Dalton Ross, assistant managing editor for Entertainment Weekly magazine, sat down with Early Show co-anchor Julie Chen to discuss the highly-anticipated finales, as well as his top three best and worst series finales in TV history.
According to Ross, a "happily-ever-after" ending just isn't in the cards for "24's" Jack Bauer, and that "the show is ultimately a tragedy."
"Well, Jack Bauer always pays a price at the end of every day (every season) for either crossing the line or breaking the rules or messing with the wrong person," he explained. "But that's what he does, that's what we all have grown to like about him. So I expect that to be what he does in the series finale."
Bringing Down the Curtain on Beloved Series
So how does Ross envision this series ending for Jack Bauer and company?
"I think it's not going to end well ... he will make it out alive, he will live to save another day because there is a '24' movie in the works. So Jack Bauer will not end in such a good place, but it won't be six feet under," he said.
Will fans be pleased with the ending?
"I don't think fans will be mad. If you're a fan of the show, you have to almost expect something bad to happen to Jack Bauer. If at the end of the series everyone's hugging and happy and popping champagne, people will be mad. Of course that won't happen. I think fans are ready for and expecting a bleak ending," Ross added.
Now onto "Lost," which has kept fans guessing and at the same time making them crazy trying to unravel its many confusing layers for the last six seasons.
What should "Lost" fans expect for the series finale?
"I can guarantee it's going to be really confusing," he said. "I think we'll all be scratching our heads at the end of the 2-1/2 hour finale. And there will be 10 million theories about what it all means. I think this show will go down as the most picked-apart and scrutinized show in history."
"Lost" fans are very passionate and extremely critical and will dissect everything about the series finale.
"'Lost' has lost its way along the way and they've caught heat for that. But I will say, the writers have a tradition of sticking the landing when it comes to season finales, so I'm pretty sure they've got something good in store for the series finale," he said. "It's such a fine line they have to walk to make it smart and clever, and not out of this world."
(The "Lost" series finale airs this Sunday, May 23. The "24" series finale airs Monday, May 24.)
TV series finales often hit a chord with many viewers and are a significant part of pop culture history.
According to Ross, his three best series finales are:
• #1: "Newhart"
• #2: "The Shield"
• #3: "The Sopranos"
His three worst are:
• #1: "Seinfeld"
• #2: "St. Elsewhere"
• #3: 'Roseanne"
Ross and Chen had to touch upon the series finale of "M*A*S*H," which was the most-watched series finale in television history.
When "M*A*S*H" went off the air in 1983, more than 100 million people tuned in, making it the most-watched program ever (until 106.5 million people tuned in for the 2010 Super Bowl).
Although some people thought the "M*A*S*H" series finale "was overblown and weird and out there," Ross points out that "it certainly took risks."
"Obviously it resonated with fans, since it was watched by over 100 million people. I don't know if, creatively, it goes down as the best series finale in history, but it was good," he said.