FFT with day-periodic data I have a collection of data: number of hours per day, over the course of several years.  I want to create an FFT of this data.  Since my sampling rate isn't seconds, but days, I think I have two choices when constructing the FFT.  One, I can specify that the sample rate Fs=1/86400 samples per second (86400 seconds in a day.)  But I think that's not right.  So maybe I can: two, say Fs=7 samples per week, then my resulting FFT has a scale, instead of cycles-per-second, of cycles-per-week.  Is this reasonable, or is there a better way to go about doing FFT for daily-periodic data?
Saying "7 samples per week" is perfectly reasonable.  The FFT doesn't care how the rate is defined, it only requires it to be constant.

When I fiddle with really low rates, I use the same concept.  Say, samples per minute or samples per year.  

Mathematically, it doesn't matter.  You know yourself that the number 1/86400 samples per second is mathematically the same as 7 samples per week - you calculated them both, after all.

The only effect it will have is in how you translate the FFT bins into a frequency - and you have to do that anyway, regardless of how you represent the sampling rate.

For myself, I find it easier to think of the numbers and samples when represented as some whole number of samples per time unit.