I forget how to do this too (it is in the instructions) as I use all hard wired motion sensors but the following information should help if you want to permanently disable this feature which a lot of people consider to be a nuisance instead of a feature.

Disabling dusk/dawn sensor

Open the battery compartment and remove the two (2) AAA batteries.

Remove the 4 screws on the back 4 corners and separate the top and bottom halves of the enclosure.

Locate the sensor. Toward the center of the circuit board you will see 2 sensors. The motion sensor is the round "can-like" object in the middle of the circuit board with a square glass-like window. The Dusk/Dawn sensor is located just down and to the right of this - See Figure 1

Use either an electrician tape or heat-shrink tubing and seal the to light.

Alternatively: To completely disable the MS-12A Dusk/Dawn sensor and save battery life, disconnect the entire dusk dawn circuit from the power supply. Remove the sensors CD1 and the two transistors TR2 and TR3 and resistors R11, R12, R13, R8, R14 and diode D1.

The above information is from [Link: geocities.com]

Ido's site has a wealth of other information too - especially for folks in countries that use 50hz 220 volt power.

Cheers, Jim.