It’s different for everyone.
If you have a 27-day cycle, it’s also possible that your ovulation happens earlier, e.g., on day 12 calculated from the first day of your period!
Sometimes you can notice that your ovulation is approaching by:
Cramps lower down in your belly.
But also because you have more vaginal discharge and stretchy mucus!
If you have stretchy mucus, it’s a great time to have sex if you want to get pregnant!!
But the online calculator is quite general, so I wouldn’t rely on it too much!!
Otherwise, just have sex whenever you feel like it.
I usually feel more like having sex during my fertile days!!
It’s also because I have more stretchy mucus then and am therefore more moist!