The season of the year affects plant growth because of light, temperature, day length, etc.
The maturity time of lettuce is doubled and tripled for plantings from September through February. To harvest lettuce every week from early November through April, the following (under cover) planting schedule is necessary:
Sept 1-10: sow every 3.5 days
Sept 10-18: 2 days
Sept 18-Oct 10: 3.5 days
Oct 10-Nov 15: 7 days
Nov 15-Dec 15: 10 days
Can be speeded up if transplants grown under lights for 3 weeks. Outdoor production has similar variables. In Maine, lettuce sown in a cool greenhouse March 1 and transplanted outside April 21 was ready for sale May 25, whereas lettuce sown April 1 in the same greenhouse was transplanted outside May 1 and ready for sale June 2.
(New Organic Grower p47)