Theme: Ignition! - A countdown to a great Monday. See Answer Grid.
21A. They start with "hip hip" : THREE CHEERS
29A. Closely related duo : TWO OF A KIND
48A. Single file : ONE AT A TIME
56A. Weightless state : ZERO GRAVITY
69A. London lav (Diagonally from square 69 to square 45) What happens after the starts of 21-, 29-, 48- and 56-Across : LOO
Argyle control here. We have lift off. Also, triple-seven's in each corner.
Across:
1. Mournful tune : DIRGE
6. Teacher's favorite : PET
9. Part of A.D. : ANNO
13. How fritters are fried : IN OIL. Yum! County fairs are starting up.
14. Bronx third baseman, to fans : A-ROD. As we learned the other day, upon coming to the Yankees, Alex Rodriguez switched positions from shortstop to third base, because Derek Jeter was already at shortstop. He also had to switch uniform numbers; he had worn 3 his entire career, but that number is retired by the Yankees in honor of Babe Ruth.
16. In __ of: replacing : LIEU
17. Diver's apparatus : SCUBA
18. Cruise ship stop : ISLE
19. Fastener in the catalog a bit above a U-bolt? : T-NUT
20. Cub Scout group : DEN
24. Russian Peace Nobelist Sakharov : ANDREI. 1975 For his work on human rights in Russia.
26. Dawn to dusk : DAY
27. Luau bowlful : POI
28. Words of understanding : "I SEE"
33. Jock's antithesis : NERD
34. Granola grain : OAT
35. In need of a massage, perhaps : TENSE
36. Candor : SINCERITY
40. Did electrical work : WIRED
43. Disencumber (of) : RID
44. Snuffs out, mob-style : OFFS
51. Chilly, in Mexico : FRIO
52. Many times o'er : OFT
53. Sidewalk stand soft drink : ADE
54. Church gathering : SOCIAL
60. Conclusion : END
61. "Dies __": Latin hymn : IRAE
62. "Wild Blue Yonder" mil. group : USAF
63. Vivian of "I Love Lucy" : VANCE
65. City west of Tulsa : ENID
66. Chow __: noodle dish : MEIN
67. Like senior statesmen : ELDER
68. Reagan and son : RONs
70. Words of agreement : YESes
Down:
1. Contempt : DISDAIN
2. Fragrant ceremonial smoke : INCENSE
3. More spherical : ROUNDER. Or the Wikipedia disambiguation page.
4. Brit. territory with a famous rock : GIB. (Gibraltar)
5. Tickle pink : ELATE
6. Duo : PAIR
7. Highland tongue : ERSE
8. City on Lake Erie : TOLEDO
9. Old, in German : ALTE
10. Bowler's back-row target : NINE PIN
11. Nervous system units : NEURONS
12. Alfresco : OUTSIDE
15. Evening coffee choice for many : DECAF
22. Think of, as a solution : HIT ON
23. Big name in hotels : HYATT
25. Obstacle for Moses : RED SEA
30. WWII GI Jane : WAC. Women's Army Auxiliary Corps (WAAC) was converted into the Women's Army Corps (WAC) in 1943. The story.
31. '90s "SNL" regular Cheri : OTERI. Maybe a puzzle with both names later.
32. It has no sharps or flats, with "the" : KEY OF C
37. Dog collar attachment : ID TAG
38. Backboard attachment : RIM
39. In other words, in Caesar's words : ID EST
40. More dizzy : WOOZIER
41. Hot spot for Dante : INFERNO
42. Teach new job skills to : RETRAIN
45. Facebook connections : FRIENDS
46. Bride-to-be : FIANCÉE
47. Joins metal : SOLDERS
49. Tight as __ : A DRUM
50. Prickly-leaved plant : TEASEL. Also, teazel, teazle, teasle or tea'sle.
55. Yiddish "Yikes!" : "OY VEY!"
57. Ref. works that occupy entire shelves : OED's. The Oxford English Dictionary.
58. Sony laptop brand : VAIO
59. "__ One Will Listen": Kelly Clarkson song : IF NO. Clip(4:00)
64. Pub potable : ALE
Argyle
Constructor's note:
This is probably the most difficult puzzle I have constructed so far. In addition to the four theme entries, there were six other entries that were restricted by the secret message hidden in the grid. The stacked triple sevens in each corner were also a consequence of the message, resulting in a rather low word count for a Monday. I’ll be curious to hear about your solving experience!