Theme: Oh, Joy! - The last word in happiness, or the last word in the entries can follow the word "happy".
20A. Guy for whom glasses are raised : MAN OF THE HOUR
37A. One-eighty : ABOUT-FACE
43A. The cellar, sportswise : LAST PLACE
58A. Fairy tale outcomes, or, in a way, what the last words of 20-, 37- and 43-Across are : HAPPY ENDINGS
Argyle here so put on your happy face and spend a happy hour in this happy place (if I can find enough links) and may everything end happily.
1. The "A" in DNA : ACID. (deoxyribonucleic acid)
5. Go on enthusiastically : GUSH
9. Drawn-out assault : SIEGE
14. Stories from the ancients : LORE
15. One of Pittsburgh's three rivers : OHIO. Allegheny and Monongahela rivers merge to form the Ohio. map
16. Open-mouthed : AGAPE
17. "Will you let me finish?" : "AHEM!"
18. Prefix with correct, in texting : AUTO. That's auto-correct, in theory.
19. Upper body : TORSO
23. Baby's bodysuit : ONESIE. Brings out the oohs and aahs.
24. Swiss river : AAR. Born from the great Aar Glaciers of the Bernese Alps, it rises and ends entirely within Switzerland. Wiki. If you look at the site, check out the awesome Aar Gorge photo.
25. Tolkien's Treebeard, for one : ENT
28. McEntire of country music : REBA
31. Carpenter's tool : HAMMER
33. Dallas NBAers : MAVS. The Dallas Mavericks play at the American Airlines Center, commonly called the AAC. That would pair nicely with the AAR.
39. Ta-ta, in Tours : ADIEU. Bye-bye, in Bordeaux.
41. Like a rotten apple : BAD
42. "Fear of Flying" author Jong : ERICA
45. Piggy bank opening : SLOT
46. Andre with eight Grand Slam wins : AGASSI. His middle name is Kirk.
47. Singer Kristofferson : KRIS. Who will nit about KRIS being part of the clue? Not I.
50. "You got it" : "YES!"
51. Bag-screening org. : TSA. (Transportation Security Administration)
53. Bahamian capital : NASSAU. Officially the Commonwealth of The Bahamas.
61. Cast member : ACTOR
64. Govern : RULE. The Bahamas is a Commonwealth realm with Queen Elizabeth II as head of state.
65. Notes before mi : DO RE
66. Friars Club event : ROAST. Manac made a timely post last night.
67. "Why would __ to you?" : I LIE
68. Washstand pitcher : EWER
69. Pear centers : CORES
70. Show the way : LEAD
71. They can get rid of the gray : DYES. Or not all of it, Touch Of Gray®.
1. San Antonio tourist spot : ALAMO
2. Broadway composer George M. : COHAN. An early hit version of "The Yankee Doodle Boy", was recorded by Cohan's contemporary and fellow Irish-American Billy Murray, 1906. Clip(4:56) Well worth a listen. If you have the time(I don't right now), you can read about "Little Johnny Jones", the Cohan musical the song came from. Link
3. Cara or Castle : IRENE
4. Floor models : DEMOS
5. Colonel Sanders trademark : GOATEE
6. "No way" : "UH UH"
7. Set of Web pages : SITE
8. Big commotion : HOO-HA
9. Waterlog : SATURATE
10. Composer Stravinsky : IGOR. A contemporary of Cohan, yet worlds apart.
11. Corn purchase : EAR
12. Some family docs : GP's. (general practitioner)
13. Fair-hiring letters : EEO. (equal employment opportunity)
21. Cedar relative : FIR
22. Diamond Head's island : OAHU. Hawaii
25. Online letter : EMAIL
26. Big name in candy wafers : NECCO. (New England Confectionery Company)
27. Pick up the tab : TREAT
29. Gilda Radner's __ Wawa : BABA
30. Taken __: disconcerted : ABACK
32. GE and GM, for two : MFR's. (manufactures)
33. Southeast Asia's __ Peninsula : MALAY
34. Proverb : ADAGE
35. Passport stamps : VISAs
36. Adjusts, as an alarm clock : SETS
38. German border river : ODER
40. Presumptuous nouveaux riches : UPSTARTS
44. Speech imperfection : LISP
48. Requiring assistance : IN NEED. A friend in need is a friend indeed. Link to The Phrase Finder.
49. Tear-jerking : SAD
52. Showery month : APRIL
54. Allied oneself (with) : SIDED
55. Like many New England winters : SNOWY
56. Shake on it : AGREE
57. Icon clickers : USERS
58. Garden waterer : HOSE
59. Santa's busy season : YULE. And how!
60. Director Kazan : ELIA
61. Circle segment : ARC
62. Loving murmur : COO
63. __ Heel: North Carolina native : TAR