Canada has suffered through nine elections in June, five of which might’ve technically counted as summer votes. But if you follow the school year definition, there’s been three summer elections—July 1930, August 1953 and July 1974.
In two of those, turnout was down from the previous election, but in 1930 it actually went up, from 67.7 in 1926 to 73.5.
In 1930, King’s Liberal government was thrown from office in favour of RB Bennett’s Conservatives. In 1953, St. Laurent’s Liberal government lost 22 seats but maintained a large majority. In 1974, Trudeau’s Liberal government won 32 more seats to go from a minority to a majority.
Historical precedent then is probably of very little use.