Entry in progress—B.P.
     
Wikipedia: Showtime at the Apollo
Showtime at the Apollo (formerly It’s Showtime at the Apollo and Apollo Live) (originally syndicated) is a music television show, first broadcast on September 12, 1987 to May 24, 2008 with 1093 episodes, and is produced by the Apollo Theater. In 2018, the series returned on Fox with Steve Harvey hosting. The show features live performances from both professional and up-and-coming artists, and also features the Amateur Night competition made popular at the famous Apollo Theater in the Harlem neighborhood of Manhattan in New York City, where the show is recorded. The series was rebooted as Apollo Live and hosted by comedian Tony Rock. In many cities such as New York (where it aired on WNBC), it often aired after Saturday Night Live on early Sunday mornings, and was often paired with the similarly-syndicated Soul Train.
 
(Oxford English Dictionary)
showtime, n.
The time at which an entertainment (such as a play, film, concert, etc.) begins or is scheduled to begin. it’s showtime: an exclamation signalling this.
1893   Fall River Line Jrnl. 30 Oct. 5/1   She [sc. a performer]..told me not to wake her until half an hour before showtime.
1941   Abilene (Texas) Reporter-News 31 July 14 (advt.)  Go to a movie. It’s showtime.
       
Newspapers.com
5 March 1955, New York (NY) Age Defender, “Front and Center” by Edward (Sonny) Murrain, pg. 17, col. 1:
So Studio Films, rather than lose its shirt in the venture, is about to peddle “Showtime at the Apollo” to a syndicate which operates a slew of theatres in Negro-populated areas of the South.
 
OCLC WorldCat record   
It’s show time from Hollywood. Program FD-173 ; It’s show time from Hollywood. Program FD-174.
Author: Freddy Martin; Charles Bilek; Stuart Wade, (Vocalist); Barclay Allen; Clayton Cash; All authors
Publisher: Cincinnati, Ohio : Frederic W. Ziv Co., [1956]
Edition/Format:   Music LP : English
 
Newspapers.com
5 October 1963, Daily News (New York, NY), “Television Programs,” pg. 30, col. 1:
Showtime at the Apollo; Nat King Cole, others.
   
OCLC WorldCat record
It’s Show Time Again
Author: B Maddams
Edition/Format: Article Article : English
Publication: The Cactus and Succulent Journal of Great Britain, v30 n3 (19680801): 54-55
 
OCLC WorldCat record
Van Morrison: Gonna Rock Your Gypsy Soul
Author: Richard Williams
Edition/Format: Article Article : English
Publication: Melody Maker, July 1973
Summary:
“IT’S SHOWTIME, ladies and gentlemen! And here’s the one you’ve been waiting for the Caledonia Soul Orchestra with ... VAN MORRISON!”
     
OCLC WorldCat record
It’s show time, Snoopy
Author: Charles M Schulz
Publisher: New York : Fawcett Crest, [1975].
Series: Fawcett Crest Book.
Edition/Format:   Print book : English
 
OCLC WorldCat record
It’s showtime.
Author: Richard Gertner
Publisher: [S.l. : s.n.], 1976.
Edition/Format:   Article : English
Publication: Motion picture product digest, Vol. III, No. 23 (14 april. 1976), p. 91
   
14 May 1985, Newsday (Long Island, NY), “Showtime Again On 125th Street: The Apollo Is Back In Orbit” by Paul D. Colford, pt. 2, pg. C4, col. 1:
“LADIES AND GENTLEMEN, it’s showtime at the Apollo!”
 
From 1934 to 1978 and a few times thereafter, that introduction heralded revues featuring Billie Holiday, Sam Cooke, Aretha Franklin, the Temptations and other performing greats. In Harlem, the Apollo Theatre reigned.
 
YouTube
IT’S SHOWTIME!
Jan 26, 2011
hh1edits
Supercut featuring various ‘Showtime’s’. List of films: https://www.pajiba.com/guides/its-showtime-a-video-by-harry-hanrahan.php
 
Pajaba
It’s Showtime!
By Dustin Rowles | Guides | April 22, 2011
Harry Hanrahan, who has been busy this month, makes another return with this amazing supercut, a companion piece to his “Get Out of There!”
 
I think this one is fairly self-explanatory. Showtime begins when you click play.
 
YouTube
It’s Showtime At The Apollo (Promo)
Aug 16, 2016
CAEG Films
It’s Showtime At The Apollo!  Produced live from the legendary Apollo Theater in Harlem, New York; an American entertainment institution since 1913.  The Apollo Theater is famous for delivering the rich sounds of jazz, soul, hip-hop, R&B, Gospel, Rap and Rock-n-Roll to the Apollo Theater stage. In addition, Apollo’s world famous Amateur Night where gutsy performers let the world’s toughest audience decide their fate, has launched some of the biggest stars of stage and screen.  A