How will i know when was my fertillity date.?

mam di po ba safe ka before and after my piriod.kasi po iregular po ako ang gnagawa ko po for example my piriod start aug.20 and ends minsan 3days lang wala or 4days wala na ko and end my piriod 24.from end of my piriod kailan po ko maging fertile paano po ako magstart mag bilang.