“Summer stock” are stage productions that are performed during the summer. Many New York actors have practiced their craft in the small cities that host summer stock productions. “The Summer Stock Company played ‘A Celebrated Case’ to a .small audience last Thursday evening” was cited in an 1884 Wisconsin newspaper. “The first performance of ‘The Danites’ by the opera house summer stock company will occur tomorrow evening” was cited in an 1885 Ohio newspaper.
 
The Cleveland (OH) Plain Dealer noted in 1887:
 
“Several of the large cities in the country have such companies during the summer season, and I have noticed of late that even the small towns are beginning to have their summer companies, composed of people who have finished their regular seasons. Cleveland may not take kindly to such a venture; but it might be worth while to try the experiment and one or two performances will clearly indicate the success or failure of a local summer stock company.”
 
Summer stock has also been called the “straw-hat circuit.”
 
     
Wikipedia: Summer stock theatre
Summer stock theatre is any theatre that presents stage productions only in the summer. The name combines the season with the tradition of staging shows by a resident company, reusing stock scenery and costumes. Summer stock theatres frequently take advantage of seasonal weather by having their productions outdoors or under tents set up temporarily for their use.
 
Some smaller theatres still continue this tradition, and a few summer stock theatres have become highly regarded by both patrons as well as performers and designers. Equity status and pay for actors in these theatres varies greatly. Often viewed as a starting point for professional actors, stock casts are typically young, just out of high school or still in college.
 
History
Summer stock started in 1919-1920s with four theatres: The Muny, St. Louis, Mo. (1919) is the nation’s oldest and largest outdoor musical theatre; Manhattan Theatre Colony, first started near Peterborough, New Hampshire (1927) and moved to Ogunquit, Maine; the Cape Playhouse, Dennis, Massachusetts (1927); and the Berkshire Playhouse, Stockbridge, Massachusetts (1928). Many of the theatres of the heyday, the 1920s through the 1960s, were in New England. Part of the “straw hat circuit,” theatres also were in New York, Pennsylvania, and Ohio, among other states.
   
Merriam-Webster Dictionary
summer stock
noun
Definition of SUMMER STOCK
:  theatrical productions of stock companies presented during the summer
First Known Use of SUMMER STOCK
1927
 
(Oxford English Dictionary)
summer stock n. U.S. theatrical productions by a repertory company organized for the summer season, esp. at holiday resorts, freq. attrib.
1942   L. V. Berrey & M. Van den Bark Amer. Thes. Slang §587/4   Straw hat, a summer stock theater, in which plays are tried out.
1955   J. P. Donleavy Ginger Man vii. 64,  I was once approached by a talent scout in summer stock.
 
8 July 1884, The Lake Shore Times (Manitowoc, WI), pg. 3, col. 2:
The Summer Stock Company played “A Celebrated Case” to a .small audience last Thursday evening.
     
26 May 1885, Cleveland (OH) Plain Dealer, pg. 3, col. 4:
The first performance of “The Danites” by the opera house summer stock company will occur tomorrow evening.
 
5 June 1887, Cleveland (OH) Plain Dealer, “Plays and Players,” pg. 3, col. 3:
“Several of the large cities in the country have such companies during the summer season, and I have noticed of late that even the small towns are beginning to have their summer companies, composed of people who have finished their regular seasons. Cleveland may not take kindly to such a venture; but it might be worth while to try the experiment and one or two performances will clearly indicate the success or failure of a local summer stock company.”
   
Variety archives
19 May 1906, Variety:
Stanley Murphy, who has been with the “Maid and the Mummy,” is about to take the vaudeville plunge instead of his usual summer stock engagement.
   
Variety archives
4 August 1906, Variety:
The last week of burlesque by the summer stock company is announced by Manager Fennessy.
   
Variety archives
28 March 1913, Variety:
The Shuberts arc planning to lease at least six of their theatres in different sections of the country for summer stock, turning them over the second and third week in April.
 
OCLC WorldCat record
Carol plays summer stock
Author: Helen Dore Boylston
Publisher: Boston : Little, Brown and Co., ©1942.
Edition/Format:   Print book : Fiction : Juvenile audience : English : 1st ed
 
Wikipedia: Summer Stock
Summer Stock, known as If You Feel Like Singing in the UK, is a Metro-Goldwyn-Mayer musical made in 1950. The film was directed by Charles Walters and stars Judy Garland and Gene Kelly and features Eddie Bracken, Gloria DeHaven, Marjorie Main, and Phil Silvers. Nicholas Castle Sr was the choreographer.