Not everyone gets egg-white-cervical-mucus, otherwise known as "EWCM", and you don't need it to conceive--it just increases your odds of conception because the sperm can live longer in it.

When EWCM does occur, it can happen on and off for days or even weeks before ovulation, depending on the cycle length. On the average, it can occur one to six days before ovulation, and while most women experience a drying of CM once ovulation is occuring or has already occured, some women do have EWCM or creamy-CM during and/or after ovulation. Everyone is a little bit different, and cycle patterns can change for the individual woman, too, so it's pretty variable.

The last day you see your most fertile CM of any cycle is considered to be your "peak day of fertility" (PDOF). When only judging by CM, you need 5 'drier-type-CM' days in a row to confirm that ovulation probably occured about 6 days prior. (When in doubt, BabyDance, [BD], I.E., have sex! ;D)

The only thing that overrides seeing the last day of your most fertile CM that cycle as being your PDOF  is feeling EWCM Sensation and/or have spotting.

If you feel this and/or see spotting, the last day (and that could be the only day) you feel it is then considered your peak day of fertility. Ovulation generally takes place within one to three days of your PDOF and rarely, up to 5 days after your peak day of fertility.

When trying to conceive it is probably best to BD on any day that isn't dry or sticky (CM-wise), any day you feel EWCM Sensation, like when AF is not in town, but it sure feels like she is, and/or any day you see spotting.