Day1-5 ok right away
You are protected against pregnancy
right away if you take it during the first 5 days after your period starts.
So if your period starts on monday, you can start the pill up to friday for instant protection. If you start on day #6 or later, then it takes about a week before you're fully protected & so you should use a back up method (condom, female condom, diaphragm, or sponge) during the 1st week.