How you can say that demand for labour in many industries was seasonal?

In many industries the demand for labour was seasonal. Gas works and breweries were especially busy through the cold months. So they needed more workers to meet their peak demand. Book- binders and printers, catering to Christmas demand, too needed extra hands before December. At the waterfront, winter was the time that ships were repaired and spruced up. In all such industries where production fluctuated with the season, industrialists usually preferred hand labour, employing workers for the season.