Blue ice occurs when snow falls on a glacier, is compressed, and becomes part of the glacier. Air bubbles are squeezed out and ice crystals enlarge, making the ice appear blue.

Small amounts of regular ice appear to be white because of air bubbles inside them and also because small quantities of water appear to be colourless. In glaciers, the pressure causes the air bubbles to be squeezed out, increasing the density of the created ice. Large quantities of water appear to be blue, as it absorbs other colours more efficiently than blue. A large piece of compressed ice, or a glacier, similarly appears blue.
Bill is going to see some sites this week. On Wednesday he is going to see Fox Glacier, and on Thursday he is going to see Cobra Lake, which has frozen over for the winter. He has been excited to see all the sites.
On which day will Bill most likely see blue ice?