Theme: No Reveal Monday - Start with three letters; a vowel progression and the same two consonants.
17A. Hebrew or Latin, e.g.: ANCIENT LANGUAGE
23A. Meetings of self-improvement seekers: ENCOUNTER GROUPS
39A. Form 1040 and schedules: INCOME TAX RETURN
53A. Doctor with a pager: ON-CALL PHYSICIAN
61A. Most of the Atlantic, to Columbus: UNCHARTED WATERS
Argyle here. The kicker is the five entries are all grid spanners and in language. Well done!
Across:
1. Contractor's detail, briefly: SPEC. (specification)
5. NYSE listings: COs. New York Stock Exchange/companys
8. Shade-loving plant: HOSTA
13. Pull's opposite: PUSH
14. Sitting on: ATOP
16. "That's __!": "Piece of cake!": A SNAP
20. Cpl., for one: NCO. Corporal/non-commissioned officer
21. Sundial number opposite I: VII
22. Kitty coat: FUR
29. Child of a boomer: Xer. Generations
30. With 31-Across, flying exhibition: AIR
31. See 30-Across: SHOW
32. More than fair, less than great: GOOD
34. Petting __: ZOO
36. Composer Bartók: BÉLA
44. Right triangle ratio: SINE
45. "Impractical Jokers" network __TV: TRU. truTV.
46. Most fit to be drafted: ONE A
47. Captures: NABs
50. Plus: AND
52. "CSI" evidence: DNA
58. Itinerary word: VIA. (by way of)
59. Dubai's fed.: UAE. (United Arab Emirates)
60. Glamorous Gardner: AVA
68. Allow to board: LET ON
69. "Frozen" queen: ELSA
70. Game with rooms and weapons: CLUE
71. Natives for whom a Great Lake is named: ERIEs
72. Like a clever devil: SLY
73. Outdoor faucet attachment: HOSE
Down:
1. Massage facility: SPA
2. Joke with a homophone, say: PUN
3. Top-left PC key: ESC
4. Trouser material: CHINO. Trousers of such a fabric gained popularity in the U.S. when Spanish–American War veterans returned from the Philippines with their twill military trousers.
5. Elevate to sainthood: CANONIZE
6. Giants great Mel: OTT
7. Work a crossword puzzle: SOLVE
8. "Macbeth" cauldron stirrer: HAG. One of the three Weyward Sisters.
9. Buckeye State sch.: OSU. Ohio State University.
10. Acronym for a big mess: SNAFU
11. Prepare to advance after a fly ball: TAG UP
12. Mimics: APERS
15. Bridge partnerships: PAIRS
18. S.A. country at zero degrees latitude: ECUA. (Ecuador)
19. About to happen: NIGH
23. Army vet: EX-GI
24. Vegas signs: NEONS
25. Early spring blooms: CROCI
26. Easy run: TROT
27. Hanger near the shower: ROBE
28. Be a debtor of: OWE TO
33. Summer of disco: DONNA
35. Cereal grain: OAT
37. Monday, in Metz: LUNDI. French.
38. "Ben-Hur" setting: ARENA
40. Dinner, e.g.: MEAL
41. Like Superman's special vision: X-RAY
42. Flees: RUNS AWAY
43. Indian flatbread: NAAN
48. "It all happened so fast" memory: BLUR
49. Sudden burst: SPATE
51. Operatic icon: DIVA
53. Developing egg: OVULE
54. Forty-__: NINER
55. Desert plants: CACTI
56. Shoes that make you look taller: HEELS
57. Hidden downside: CATCH. There's always a catch.
62. Soil-moving tool: HOE
63. Ques. response: ANS. (answer)
64. Broadband letters: DSL. (Digital Subscriber Line)
65. Rock gp. with winds and strings: ELO. (Electric Light Orchestra)
66. Moscow's land: Abbr.: RUSsia
67. Lay eyes on: SEE
Argyle