Ramadan is approaching and it is good to know when to make up any missed fasts and when you should pay fidyah.
If you have an excuse in Ramadan to miss any days of fasting then you should make up those fasts as soon as you can. You should make them up before the next Ramadan.
If you are not fasting for a legitimate reason and you are not going to make up the fast then you must pay the fidyah or kaffarah on that very day. You can also prepay or pay at the end of the month. It should be done during that month as soon as possible.