how did the growing seasons mark time on the calendar for the farm workers?
Without knowing what work you're reading, I'm assuming you have encountered this description in your novel. What the author is saying is that for farm workers, particularly for migrant workers, the division of the year into months shown on a calendar isn't as important as the progression of growing seasons in the fields.
Based on the weather, farm workers plant seeds for new crops when the soil temperature and moisture is optimal for the sprouting and growth of new plants. The actual date on the calendar is not as important as the growing conditions.
Harvest depends upon the weather and growth and development of the crops, not a specific day or week of the calendar. Because different crops require different lengths of time to mature, migrant farm workers move from harvesting a crop that grows in a shorter length of time to a crop that needs to grow for a longer period. The ending of the growing season for each type of crop determines when they move from one harvest to the next, not a calendar.