The problem is that the constant offset value of your signal is very high compared to the amplitude of the oscillations. I would remove the constant offset (the mean of your data) before calculating the Fourier transform. That will show the oscillations much more clearly.
Example —
n = length(H);
Ts = 1; % Sampling Interval (day)
Fs = 1/Fs; % Sampling Frequency (samples/day)
Fn = Fs/2; % Nyquist Frequency
nfft = 2^nextpow2(n);
Hn = H-mean(H); % Remove Constant Offset
ft = fft(Hn,nfft)/n;
Fv = linspace(0, 1, fix(n/2)+1)*Fn; % Frequency Vector
Iv = 1:length(Fv); % Index Vector
figure(1)
plot(Fv, abs(ft(Iv))*2)
grid
I also provide the frequency vector for your data, and plotted it with respect to that.
NOTE — I do not have your data, so this is UNTESTED CODE. It should work.
