Not suitable to or appropriate for the season.(adjective)
Not characteristic of the time of year: unseasonable weather. (adjective)
Poorly timed; inopportune.(adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "unseasonable" in a sentence
"It was one of those days in June, in which our summer-hopes take umbrage at what we call unseasonable weather, though no season was ever known to pass without them."
"This hour seemed to him and to Mrs. Peterkin unseasonable, at a time of year when the sun was not up, and he would have been obliged to go to the expense of candles."
"Rene Heroux, a meteorologist with Environment Canada, called the unseasonable weather a heat wave."