Replacing all NAs with smoothing spline

Replacing all NAs with smoothing spline

Below is the sample data (out of approximately 8000 rows of data). How can
I replace all NAs with values from a smoothing spline fit to the rest of
the data?
Date Max Min Rain RHM RHE
4/24/1981 35.9 24.7 0.0 71 37
4/25/1981 36.8 22.8 0.0 62 40
4/26/1981 36.0 22.6 0.0 47 37
4/27/1981 35.1 24.2 0.0 51 39
4/28/1981 35.4 23.8 0.0 61 47
4/29/1981 35.4 25.1 0.0 67 43
4/30/1981 37.4 24.8 0.0 72 34
5/1/1981 NA NA NA NA NA
5/2/1981 39.0 25.3 NA NA 55
5/3/1981 35.9 23.0 0.0 68 66
5/4/1981 28.4 22.4 0.7 70 30
5/5/1981 35.5 24.6 0.0 47 31
5/6/1981 37.4 25.5 0.0 51 31