Sorry Jeff - should have responded. We basically have 3 days in the season where stats are taken of each teams financial situation. As Zach correctly pointed out, your top 21 OV players count towards your cap. There are minimums for player categories, 3 centers, 3 LW, 3 RW, 5 D and 2 G. After those 16, the next 5 highest OV regardless of position or roster spot (i.e. minor league players still count.)
The first day was the first day of the season. The next day approaches soon (in about 8 games on average... after all teams have 30 games and before they have 36). The final day is the day after trade deadline day.
When I work out my cap I add up my top 21 salaries at each point and divide by 3 to get my season average.
If I run at 50 million in each of the first two checkpoints I can in theory carry players salaries well over the cap at the third period, because my average over the season will still be low. In fact I've done it once and a lot of teams do it every year.