The Minima of Algol
Algol fades and rebrightens like clockwork every 2.87 days. Its changes are very plain to the naked eye. In the middle of an eclipse it shines dimly at magnitude 3.4 instead of its usual 2.1.
Algol stays nearly that faint for two hours centered on the time of mideclipse, and it takes several additional hours to fade and to rebrighten.
Good comparison stars are Gamma (γ) Andromedae to Algol's west, magnitude 2.1, and Epsilon (ε) Persei to its east, magnitude 2.9. You can compare Algol's brightness with them at a glance; click on the star chart to see a larger version with the magnitudes of several comparison stars clearly labeled. (For detailed tips on estimating a variable's brightness, see "The Lure of Variable Stars." For information and finder charts for Algol and 11 other inconstant stars, see "The Top 12 Naked-Eye Variable Stars.")
Below is a calculator you can use to predict when Algol will be at mideclipse. Press "Initialize to today" to view the dates and times of Algol's minima for the next three weeks. Or you can enter any date, between 2008 and 2015 inclusive, to see the dates and times of eight consecutive minima. The times given should be accurate to within a few minutes.