Do numbers, letters or days of the week have colours or smells? They might if you experience the unusual phenomenon called Synesthesia. To a synesthete, one or more sensory channels cause a kind of "cross-talk" that triggers experience in another channel.
A synesthete may experience colours with numbers or letters (very common), colours with sounds or musical notes (also common), taste or touch with certain sounds (less common) or even colours with changes in temperature (more rare).
No one really knows what causes Synesthesia, but neuroscientists are fascinated by it and are conducting research to try to better their understanding. Some believe that we all started out as synesthetes, but lost the ability in early childhood.
More, from Wikipedia:
"In one common form of synesthesia, known as grapheme → color synesthesia or color-graphemic synesthesia, letters or numbers are perceived as inherently colored, while [...] numbers, days of the week and months of the year evoke personalities. In spatial-sequence, or number form synesthesia, numbers, months of the year, and/or days of the week elicit precise locations in space (for example, 1980 may be "farther away" than 1990), or may have a (three-dimensional) view of a year as a map (clockwise or counterclockwise)."
An example of how a word might be perceived by a synesthete: