#define BUTTON_PIN 3

volatile byte buttonReleased = false;

void buttonReleasedInterrupt() {
  buttonReleased = true;
}

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_PIN, INPUT);
  attachInterrupt(digitalPinToInterrupt(BUTTON_PIN),
                  buttonReleasedInterrupt,
                  FALLING);
}

void loop() {
  if (buttonReleased) {
    buttonReleased = false;
    // do something here, for example print on Serial
    Serial.println("Button released");
  }
}