Sometimes people think they can get something done by a date, genuinely believing it will be possible, but then they can't. This will not be the last time in your life that you come across this phenomenon. You are not paying her for her time spent writing your reference, so be respectful of the time that she does spend doing so, and her right to choose when she does so (always assuming the deadline is met in the end). She is giving you her time, and you must realise that she's not exactly well remunerated for doing so. The one thing that teachers do get is their holidays, so let her enjoy her last day of holiday and call her on Monday.