JOEL - The Family Home Evening program started around 1965. In 1970 the Church designated Monday night for Family Home Evening with no other Church meetings or functions planned, so that everyone in the Church would be free that night each week.
So Monday night is recommended only because there are no conflicts with any other Church meetings and Church members know to not call other members on that night. Many families choose other days of the week such as Sunday afternoon. It's up to you to choose the best time for your family if Mondays don't work well for you. The important thing is to do it consistantly.