Cut off frequency for low pass filter
The butter command expects a normalized frequency from 0 to 1, where 1 is the Nyquist frequency.  In other words, Wn is in half-cycles per sample.

So you just divide your desired frequency (1 cycle / 48 hours) by half the sample rate ((1 sample per hour)/2), giving Wn = 0.04167 half-cycles per sample.