Polynomial Regression - Advice please

Sorry if i'm posting this twice, received an error first time around. Im trying to develop some kind of statistical model that lets me estimate sales by the end of the month based off of my sales at any given point of time during the month. Ive noticed that sales trend on a polynomial curve consistently increasing rapidly as I move towards the end of the month.

I've mapped 4 years of daily sales data against business day of the month to get a sales curve. Ive then plotted a polynomial regression line over my data and used excel solver on the coefficients to get a line with an R squared of .997. Pretty good line.

However, I now realize that all I can do with my equation is predict sales based off of a day or predict working day based off of sales. It does not let me predict sales at the end of the month based of off my sales number at a point in time. I can get an approximation by taking my % increase for a historical point in time to my line apex, but I feel like there are more scientific ways of going about this.

Any advice on any techniques I can employ to achieve my goal?