Venus was the goddess of growing things, gardens, love and fertility for the Romans. People sacrificed to her when they wanted to have babies, or to make somebody fall in love with them. People thought of Venus as watery and fluid - she was pictured as being born out of the ocean foam.
The Roman name for Friday was "dies Veneris", the day of Venus. In Italian, Friday is still "venerdi", and in French it is "vendredi". In English, people translated it to the name of the German goddess Freya, so we call it Friday.