By Dan Primack
May 16, 2012

FORTUNE — Facebook (FB) is going public this week and, for many existing shareholders, it’s not a moment too soon.

If the company begins trading on Friday, it would be exactly 228 days before the end of 2012. Or, put another way, 228 days before capital gains tax rates are expected to rise.

Why does this matter? Because Facebook has instituted a series of “lock-ups” for existing shareholders, including company employees.

The vast majority of these expire 181 days after Facebook goes public, which likely means they expire on November 15. At that point, holders would have just six weeks to get liquid before 15% capital gains rates expire and are replaced by the pre-Bush 20% rates. In fact, 20% might be a floor, depending on how the elections go — given certain Democratic proposals to increase capital gains rates even more on high earners (a bucket into which most current Facebook shareholders would almost certainly fall).

“I don’t care if the IPO in May or June, so long as it isn’t in July,” said one current Facebook investor, who bought shares on the secondary market from company employees.

But not everyone necessarily feels that way. Certain Facebook shares are locked up for 211 days after the IPO, which would work out to December 17 (technically the day before, but that would be a Sunday). So, for those positions, pricing now is imperative.

In fact, the only current Facebook shareholders locked up past 2012 will be Russia’s DST Group and Mail.ru Group. Those firms, which are selling around 35% of their holdings at IPO, will be prohibited from selling additional shares until May 19, 2013.

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