This year is a leap year, giving February 29 days instead of its usual 28. This is required because the earth actually takes about 365.2422 days to go round the sun, not a simple 365. If we didn't have leap years the season's and the months would no longer match (shifting each year by about 6 hours). As it is, it's still quite complicated and the astronomers have to be on the case to know when a leap day needs to be added or left out so that we don't end up with winter in May and spring in August! See the link above for more on the maths.
So, February 29th, leap day, generally comes once every four years. Traditionally, this is the day for custom to be inverted and for women to propose marriage to their beloveds, rather than waiting for the men to take the plunge. This tradition goes back hundreds of years; back in 1256 it was decreed by English law that the leap day and the day before were to be reckoned as one day for the purpose of calculating when a full year has passed. Thus, February 29th was 'leapt over' and was regarded having no status in law and became a day when tradition could be disregarded. In some versions of the custom it became the entire leap year for which women could propose, while others held it to be only that single day.
So, ladies, there's an extra incentive to take the initiative this year. Think saying it with flowers might be too girlie for him? Actually, a recent study carried out by Rutgers University found that men, as well as women, respond to a gift of flowers with increased signs of happiness and improved social interaction. Women may show it more, but in fact the positive effect of flowers is equal for men and women, across a range of ages and ethnic groups. The study found that, compared to a control group who did not receive any, men given a surprise gift of flowers demonstrated greater eye contact and stood closer in conversation, they also smiled more and more genuinely.