Some satellites have large reflecting surfaces and under certain conditions they reflect sunlight to the earth thus rendering them visible. The shape of Iridium telecommunication satellites is pandering reflections to the earth. They can be visible even during the day but at night they are a spectacle to watch. The perfect conditions are that the sun is shining on the satellite (no no other celestial body is obstructing the suns rays), the satellite is in a perfect angle towards the observer and he looks at a dark (night) and unobstructed (not cloudy) sky.
Some of these conditions can be calculated in advance, so you know when they will happen beforehand. You need to know your position in latitude, longitude and height. To find out about those go to Open Street Map and zoom in to your position, then click on “Permalink” at the bottom right. Note the latitude and longitude appearing in the URL of the browser.
Then go to heavens-above.com where you can enter those values to define your observing position.