Top O' the Morning! The word Day "opens" up a new phrase when added it proceeds the each word of the theme answer.
17-Across. Classroom text: SCHOOL BOOK. Day School and Day Book.
28-Across. Twinkling in the night sky: STAR LIGHT. Daystar, also known as the planet Venus, and Day Light.
39-Across. Relief from the daily grind: TIME OFF. Day Time and Day Off.
48-Across. Office spot with a coffee pot: BREAK ROOM. Day Break and Day Room.
And the unifier:
63-Across. Start of a sports season, and what each half of 17-, 28-, 39- and 48-Across can have: OPENING DAY.
Across:
23. Goes on to say: ADDS.
24. Summer on the Riviera: ÉTÉ. Today's French Lesson.
25. MLB Network analyst Martinez: PEDRO. Pedro Jaime Martinez (b. Oct. 25, 1971) is a Dominican professional baseball player who had a long career in Major League Baseball. He played for the Boston Red Sox from 1998 until 2004. He also played for 4 other teams. This Friday will be his 48th birthday.
34. On bed rest, say: LAID UP.
37. Bird's crop: CRAW.
38. Colorado native: UTE. A crossword staple.
42. "__ Am": Alicia Keys album: AS I.
45. Sinus doc: ENT. As in an Ear Nose and Throat Doctor. These doctors make frequent appearances in the puzzles too.
51. Landlord's income: RENTS.
52. Tell tall tales: LIE.
53. Field of study: AREA.
59. Closed in on: NEARED.
68. Mozart's "Così fan __": TUTTE. With foreign subtitles for your reading pleasure.
69. Spill the beans: TELL.
70. Taiwanese PC maker: ACER. I learned about this PC from doing the crossword puzzles.
71. Most common roll of two dice: SEVEN.
Down:
1. Prefix with gender: CIS-. As in Cisgender, which is the opposite of Transgender. Cisgender is the term for people whose gender identity matches the sex that they were assigned at birth.
2. How software was once sold: ON CD.
4. Johnny's "The Big Bang Theory" role: LEONARD. I never watched this show, so needed perps to help with the name.
5. Was conspicuous: STOOD OUT.
6. Convent leader: ABBESS. A famous movie Abbess.
7. Language of Southeast Asia: LAO.
8. Hard punch: BLOW.
9. Many a Punjabi: SIKH. Sikhism is a monotheistic religion that originated in the Punjab region of India during the 15th Century.
10. One leading a Spartan lifestyle: ASCETIC.
11. Healthy bread type: WHOLE GRAIN. I initially tried Whole Wheat.
18. Inc., in the U.K.: LTD.
30. Big deal: ADO.
32. "Who __?!": "Join the club!": HASN'T.
33. Small sticks: TWIGS. The Water Institute of the Gulf is sometimes referred to as TWIG.
47. Debate again: RE-ARGUE.
50. Style: MANNER.
54. Camping gear brand: REI. I am not familiar with this company.
55. Inane: DAFT.
58. On __: without a contract: SPEC.
64. WSW's opposite: ENE. East-NorthEast is the opposite direction of West-SouthWest.
QOD: Lazy people tend not to take chances, but express themselves by tearing down other’s work. ~ Ann Rule (née Ann Rae Stackhouse; Oct. 22, 1931 ~ July 26, 2015, American crime writer