when do you find out what stage your in?

Stage depends on both tumor size and nodal (or more distant) involvement, neither of which can be determined before surgery. In some cases, the tumor on imaging appears so large, and the lymph nodes determined via some form of biopsy to be involved, that staging is assigned via imaging & biopsy and neoadjuvant (pre-surgery) chemo is given to shrink the tumor to a more manageable size for excision if a lumpectomy is elected, or to determine effectiveness of chemo (pathological response) whether or not mastectomy is the ultimate surgical choice.

Often, though, core biopsy alone can determine grade (as it did for your and my tumors) as well as hormone-receptor and HER2 status (as it did mine). Because of the “guesstimated” size, I was tentatively staged as IA (they assume nodes will be negative until biopsy—either sentinel node or axillary dissection—proves otherwise. The tumor equivalent of "innocent until proven guilty”). However, sometimes that information will change—there are instances in which the final surgical pathology yields different results from the initial biopsy. (In my case, the size turned out to be larger than originally thought, though still within Stage I parameters).