Max. current (continuous) is 20mA, resulting in 3.5-3.6V forward voltage.
LEDs are operated at constant current, not voltage! It is necessary, to limit the LED forward current with a resistor. The value of the resistor is easy to calculate following Ohm's law: R = U / I
For example, on a 12V power supply, U = 12-3.6 = 8.4V, I = 0.02A. The resistor in this circuit should not be less than 8.4 / 0.02 = 420 Ohm (for max. brightness). For battery operated applications, you should consider a much lower LED current, for example 3mA (equals 2.9kOhm on 12V).