That tradition of 29th February each leap year being the one day that women can propose goes back many hundreds of years to when the date had no recognition in English law - indeed the term came from the day being 'lept over' and ignored.
It was considered that as the day had no legal status, it was reasonable to assume that traditions, including the fact that it should be the man who proposes, also had no status.
Women who were concerned about being 'left on the shelf' took advantage of this anomaly and proposed to the man they wished to marry.
It was also thought that since the leap year day corrected the discrepancy between the calendar year (365 days) and the time taken for the Earth to complete one orbit of the sun (365 days and 6 hours), it was an opportunity for women to correct a tradition that was one-sided and unjust.
While many traditions have gradually disappeared, this is one that is still going strong, and there will be some nervous looking men around tomorrow....