The only way to conquer Broadway’s annual post-Labor Day slump is to have a little faith.Faith in “The Book of Mormon,” anyway: In a week when almost every show on the Main Stem saw sales drop, “Mormon” ($1,293,582) rose — and managed to top perennial smash “Wicked” ($1,253,990) and almost nick “The Lion King” ($1,299,169). The “Mormon” feat is especially impressive since its theater seats only 1,075 vs. 1,800 at “Wicked” and 1,675 at “The Lion King.” Otherwise, it was a tough week all around, with major slides posted at most of the shows on the boards. “Lion King,” “Wicked” and “Spider-Man: Turn Off the Dark” ($1,197,278), for instance, saw week-to-week sales drop by about 20%. The annual slowdown is due to Labor Day, the official end of the summer travel months. But the effects were exacerbated by the attention directed to activities organized around the 10th anniversary of 9/11. With the concurrent startup of the school year, productions with family appeal — including “Mary Poppins” ($492,889) and Daniel Radcliffe topliner “How to Succeed in Business Without Really Trying” ($757,240) — took some of the hardest hits of the frame. The overall Broadway cume fell $4.1 million to $14.3 million for the 21 shows on the boards, with attendance dipping to 165,000 at 71.7%. Still, that marked an improvement over the week ending Sept. 12, 2010, when 23 shows pulled $13.4 million. Average paid attendance fell from $91.63 to $86.92. The 19 musicals grossed $13,263,444 for 93.7% of the Broadway total, with attendance of 545,122 at 69.7% capacity and average paid admission of $86.06. The two plays grossed $1,041,765 for 6.3% of the Broadway total, with attendance of 10,448 at 78.8% capacity and average paid admission of $99.71.