Tonight, Algol, located in Perseus, will be at minimum right around midnight Eastern Standard Time. No, the star is not going to turn off and on again like a light bulb, but it will gradually dim and then brighten again, going from magnitude +2.1 to +3.4 and back to +2.1 over the course of a couple of hours. Good companion stars with which to compare Algol are Gamma Andromeda (magnitude 2.1) to Algol's right and Mirfak (magnitude 1.8) above and left of Algol. Fortunately, it will be possible to see the event in entirety thanks to the timing of the minimum. Don't want to stay up all night? Well, you can go out before bed, look at Perseus, and then get up early to see Algol at regular brightness or go out at nightfall, see Algol as normal, stay up until the minimum, and then go to bed. Try and spot the difference.
The reason for this change in brightness? Algol is not a single star, but a two star system, as many stars are. However, what sets Algol apart is that, as seen from Earth, the dimmer companion eclipses the main, brighter star. The result: a dramatic change in brightness that earned the star its name ('Algol' is Arabic for 'the ghoul') and the nickname 'the winking demon star.'
Needless to say, this change in brightness really creeped out the ancients.