Knowing when ovulation means that you know when your fertile time of the month is. The fertilisation window each month is actually quite small for women. If you are going to conceive, it is within 24 hours after ovulation.
Ovulation usually occurs approximately 14 days before your period is due ie the 2nd stage of the menstrual cycle (the luteal phase) is usually 14 days long. This is fixed for most women although it can vary between 11-16 days for some. The part of the cycle that more often varies is the first half ie the run up to ovulation or the follicular phase.
If your follicular phase is the same each month this makes knowing when ovulation occurs very easy and predictable using a calender. The problem occurs when the folicular phase of your cycle is erratic or unpredictable. Irregularity of the pre-ovulation phase are more common in young women under 25 years & in older women (over 40 years) often being shorter or longer and less predictable.
For some women the follicular phase can be very prolonged for example in polycystic ovarian syndrome or the cycle may be anovulatory (ie ovulation does not occur)
If the cycle is irregular ovulation predictor kits can be helpful if you are trying to conceive.