Switch case (millis()) UPDATED
The following code is not very &quot;clean&quot; but easy to understand. Use a variable to hold the day, you have already printed and check in which interval you are. If you change the 2 day interval print a message and set the actDay variable to the number of the day you have already printed; to avoid repprinting it again.
int daytwo = 25920; 
int dayfour = 44400;

int actDay = -1;

void loop() 
{  
  unsigned long currentMillis = millis();
  if (currentMillis &lt; daytwo) 
  {
    if ( actDay != 0 ) 
    {
      actDay = 0;
      Serial.println(&quot;dayzero&quot;);
    }
  }
  else if (currentMillis &lt; dayfour ) 
  {
    if ( actDay != 2 ) 
    {
      actDay = 2;
      Serial.println(&quot;daytwo&quot;);
    }
  }
  else 
  {
    if ( actDay != 4 ) 
    {
      actDay = 4;
      Serial.println(&quot;dayfour&quot;);
    }
  } 
}