Insurers increase the premium on policies they more you need it. If you do a start date for today or tomorrow, it will be much higher than if you get a quote to start in 21 days, this appears to be the sweet spot of where the cheapest policies are. Underwriters also have premiums based on block underwritten risk where they understand where the most risk is through the year. They have this preplanned from the previous year, as they are looking at what money is coming in relative to going out along with a myriad of other things, but as they get closer to these dates, things change and they adjust premiums to compete (or not as the case may be) with the market.