The Thirteenth Month is a turning point in a startup. It defines the two distinct stages of starting a business. The first stage begins at conceptualization and ends on the Thirteenth Month after your product or service has been on the market. At this point, the founders (and any associated high-level employees) know without a doubt where they stand and what the future of the business will be. This is why many founders stay on for fourteen months and then bail. At just over a year, everyone knows whether the company is going to make it or break it.
If you are the CEO, founder, or stockholder, you should quietly observe the Thirteenth Month. Don't alert your comrades as to your decision (you will undoubtedly cause panic), but reflect on the previous year imagining that the rest of the company's future is the same. If you found yourself scrapping for sales, minding pennies, or other negative phenomena, you should consider your exit strategy. Otherwise, you must take the Thirteenth Month to restructure your company for the second stage of the startup. You will probably find yourself reworking large parts of your infrastructure and creating others. Knowing what you needed in the past year, make a ideal plan of the business and execute it.
This is the point to spare no expense, in contrast to the first stage. The more you spend now (assuming your were right and the business has a future) the less work you will have to three years from now. Do not spend money irrationally until the Thirteenth Month, because you are only digging a deep hole for yourself. Work lean until you can assure that you will make it. So the Thirteenth Month can be like Christmas, you can finally go all out (or as much as you can borrow) and get yourself an espresso machine, an assistant, and parking spots that don't suck (well, you might want to make sure that the business's needs are taken care of first).