int lightResistor;

bool calibrate_is_finished = false;

int threshold;

void setup() {

  threshold = calibrateLightSensor(lightResistor);
}

void loop() {

  if (checkThreshold(readSensor, 120)) {
    //Lightshow
  }

}

int readSensor(int sensorPin) {
  return analogRead(sensorPin);
}
int calibrateLightSensor(int sensorPin) {
  int maxValue = 0;
  int minValue = 1023;
   while (!calibrate_is_finished) {
       int readValue = readSensor(lightResistor);

       if (readValue > maxValue) maxValue = readValue;
       if (readValue < maxValue) minValue = readValue;
   }
}

bool checkThreshold(int value, int threshold) {
  if (value > threshold) {
    return true;
  }
  else if (value < threshold) {
    return false;
  }
}