Seeing how in either case it is a Thursday, I would imagine either or would make for the same difference, but perhaps someone else has a better answer.
If that is for June of this year, it will be difficult but not impossible to find a hotel close to the land. You may wish to begin looking into sites such as price line et al, that may be your best choice at this point.
Have fun...
~~ I love you Phil and you will forever be with me! Forever my very best and most special friend! I love you to no end. ~~
I would choose the 21st only because both Socal APs will be blocked that day. On the 28th, only the lowest Socal AP is blocked. Either way it will be very crowded because of CL opening, but on the chance it makes a difference, I'd go with the 21st. Being there early before opening will be a must either day.