Jenny, don't let it upset you, I have had this occur many times since I was (probably) in my 30's! Just happens once in a while to most of us.
I advise you invest in one of these:
Amazon.co.uk User Recommendation
Just make it your first little job of the day to turn over the wooden blocks and place it where you spend the most time, or where your phone is. It could not be easier to keep track of the day or date :)
PS if you look on your computer, bottom right you will see the date, click on it and it will enlarge the details for you.