First day of your menstrual cycle is the day you start bleeding. If your bleeding started after 6 pm, the following day is considered the first day of the cycle.
How long are eggs and sperm capable of fertilising?
Eggs can be fertilised for 24 hours, but it is most probable that the fertilisation occurs in the first 8-12 hours. Sperm can stay in the female reproductive system for over 5 days, but are most capable of fertilising in the first 48 hours.