Statistical test needed (not a homework problem)

I do actual research in Physics. My question is how do I compare two random vectors - do the measurements come from the same underlying process or not? Equivalently, if I do some measurements on two different days, how do I test whether anything has changed between these two days' worth of data?

Let's say I have one independent variable and two dependent ones. The independent variable let's say has three values A,B,C and for each case I have four random samples. On day one I measure:

A: 0.0 -0.1 -2.5 0.6
B: -2.4 -2.6 2.2 0.1
C: 7.2 8.0 8.6 3.9

and on day 2 I measure:
A: 0.4 0.1 0.8 0.5
B: 3.8 1.7 1.7 4.0
C: -0.4 4.5 2.2 4.9

Null hypothesis is that nothing has changed from day 1 to day 2. How do I estimate the p-value here?

//Mods. Please feel free to delete my posts in the homework help forum. I just realized that this is where the question needs to be asked.