Answer:
Wednesday 
Explanation:
To determine which day of the week had the lowest low temperature, let's plot the temperatures on a number line:
```
 |---------------------------|---------------------|
 -5.5 -2 5
 Wed Thurs Mon
```
Looking at the number line, we can see that Wednesday had the lowest low temperature of -5.5 degrees Celsius. Therefore, Wednesday is the day of the week with the lowest low temperature.