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. The satellite will reflect light to the earth just like a cone of light by a spotlight in the dark. When the observer is in the centre of the trajectory of this reflection the satellite appears the brightest, the farther away your position is from the centre, the dimmer it is. Since the reflection passes it will appear for the observer as if the satellite becomes brighter and then dimmer again, this is what is called a “flare”.
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, or choose one of the pre-defined locations if you are observing from an urban viewpoint.
You can get predictions for the next days including the magnitude of the flare. Check the weather conditions for your site, especially look for the cloud coverage so you know if you have a chance to see the satellite or not.