If your periods are irregular, it's hard to calculate your ovulation. Generally, you'll ovulate about 14 days before your period starts. But that would be hard to figure out if your periods are irregular. There are ovulation tests you can take to pinpoint ovulation in the future, if you're trying to get pregnant.
You ovulate only one day during your cycle... you get a surge about a day before you ovulate but actual ovulation only last a day since the egg only survives for about 24 hours.
and when was your last menstrual period? Also, I know you said your periods are irregular, but do you have a ballpark figure? Like between 28 and 33 days or something like that?
Technically, if the sperm lives for 5 days (they usually don't live that long, but they can), and your egg can be fertilized for 1 day, there are 6 days you could have gotten pregnant. However, you would have had to have intercourse
around the time you ovulate. Also, there are a lot of us in this forum who are trying to get pregnant, and let me tell you, it is not as easy as we thought :-)