The mysteries that surround the CBS sitcom “How I Met Your Mother” extend to co-creator Craig Thomas’ office on the Fox studio lot. A white board on the wall that outlines the seventh season’s episodes ends with Barney’s wedding in the May finale. “___ is the bride,” the board says. You never know who’s going to walk in, and Thomas and partner Carter Bays hold tight to their secrets. That will be a pivotal episode: Not only will the ultimate bachelor come off the market, but the show has revealed it’s also the day that Ted meets his future bride — the mother that is the conceit upon which the entire show is built. That doesn’t necessarily mean viewers will meet the mother in that episode, however. Stay tuned. This has been a big year for the comedy that launches CBS’ Monday nights. Ratings are the best they’ve ever been, up 19 percent over last season, and it has the youngest average audience on the network’s prime-time schedule. “There’s almost no scientific explanation and we couldn’t have counted on that,” Thomas said. Time may make viewers more invested in the lives of Ted (Josh Radnor), horndog buddy Barney (Neil Patrick Harris), Ted and Barney’s ex Robin (Cobie Smulders) and the married couple Marshall and Lily (Jason Segel and Alyson Hannigan). Another likely contributor is the boomerang effect of syndication making more people familiar with the series. “How I Met Your Mother,” which just filmed its 150th episode, has been seen outside of prime time on local broadcast stations the past few years. Last year it was also on Lifetime, the cable network targeted at women, and this fall added FX, which is popular with young men. The FX showings began with a marathon and a clever ad campaign that pictured the cast and suggested: “Isn’t it time you made some new friends?” “How I Met Your Mother” hit the syndication market at the time there was a relative paucity of new comedies and reruns of “Friends” were getting tired from overuse. “How I Met Your Mother” is the closest TV has to a modern-day “Friends.” It started at a time, in 2005, when networks were desperate to replace that beloved NBC series and, frankly, the namesake gimmick distinguished it from other wannabes. The series opened with kids on a couch impatiently listening to narrator Bob Saget, as Ted circa 2030, explains how their parents met. That first episode began with Ted establishing a romantic connection with Robin, ending with the kicker of Saget explaining, “that’s how I met your Aunt Robin.” During an initial meeting with TV critics before the premiere, Thomas and Bays were taken aback by anger they faced about that pilot’s twist. Were they really expecting to learn the mother in the first episode? Then they realized: People cared about the characters they created. They don’t regret the structure, even if “Who’s the mother?” is no doubt the cocktail party question they’d least like to hear by now. “I always thought the frustration about it was a little misplaced,” Radnor said. “There’s so much to enjoy beyond the central conceit of the show that I always felt like, ‘Relax.’ If he meets the mother, the series is done, so if you like the series you should be waiting. Enjoy the wait. Maybe this whole series is some grand lesson in patience for people. It certainly is for Ted.” Many fans believe the mother should be revealed on the final episode. Others would like to see the future parents go through their first year of dating. This much Thomas will say: It will be one of those two possibilities. That’s why Thomas and Bays have some important meetings ahead of them this spring. The actors are signed through the end of next season (May 2013), so the producers will need to know soon if the series is stretching beyond that. Revealing when Ted would meet his future wife turned out to be liberating. “It’s kind of a momentous thing to say,” Thomas said, “because it retired one of the tricks on this show that we had milked for half a decade, which was that any girl that Ted bumped into anywhere could be the mother. Last year we said we’d done that enough.” The device the creators set up, where viewers know in the back of their minds there’s a happy ending, helps ground the show, he said. It’s comforting to know things turn out OK, and that knowledge enables writers to show the harder chapters in their lives. “How I Met Your Mother” was born of the creators’ own experiences. They moved out to Hollywood together to try and make it as they approached age 30, after having worked as writers for David Letterman. They missed New York, and were reflective of the times they had gone through in their 20s. The show’s set is a congenial one, with a calm warmth that flows through veteran director Pamela Fryman. Cast members have busy separate professional lives. Segel is a genuine movie star (“The Muppets” and “Forgetting Sarah Marshall“), Harris is a go-to awards show host and Radnor just directed his second film (“Liberal Arts“) to open at Sundance. “It’s really a well-oiled machine here,” Segel said. “You just come and do it and hang out with your friends. It kind of feels like going to summer camp every day.” The actors say they’ve been kept interested by how the show has explored storylines beyond sitcom wisecracks. Robin learned recently that she was unable to conceive and bear children. Lily is pregnant and the couple bought a house on Long Island, while Marshall experienced the death of his father. The death of Marshall’s father was one of the creators’ secrets. The news wasn’t distributed ahead of time, because some stolen scripts had found their way to the Internet before the episode aired. Segel learned as Marshall did, when Lily told him with cameras rolling. When the episode aired, Thomas’ wife, who had recently lost her mother, complimented him on writing Marshall’s shocked reaction of “I’m not ready for this.” He had to tell her that it was ad-libbed, not written. One take. Segel said it made for a better performance. Harris said he occasionally goes online the Tuesday after a show to see what fans had written and often marvels at in-depth analysis of the characters. People seem to care about the show, he said, “and that’s awesome.” “When this chapter is done we’ll all look back and marvel at it probably being the best job we’ve ever had,” he said.