one reason to dislike Matlab

[gusl]

Array indices start from 1, which means that to properly avoid fencepost errors, your code ends up looking like this:

for day=1:floor(size(p1,2)/24) %for each day
    daytotal(day) = sum(p(24*(day-1)+1:24*(day-1)+24));
end

Lots of extraneous 1s. (And that's because the data is nice: this code can't handle partial days)

I hear NumPy is a very good substitute.