By Diane Rommel Tomorrow’s the day. March 15 is the day to book in two very fun, very California, very best-summer-ever things: 1: Hike Mount Whitney, the highest peak in both California in particular and the Lower 48 in general 2: Reserve a campsite in Yosemite for the height of the summer season Of these two, Yosemite should be your first priority. Reservations for the park’s ridiculously in-demand campsites opens today at 7 a.m. Note — this doesn’t include schmancy spots like the Ahwahnee Hotel, which boasts a seriously mediocre 4.0-star on TripAdvisor while selling its top suites north of …