Best day to visit in April (Mon-Fri or Week-End) of the Happy Valley

Hello there,
I'll be in SH next April and I'm planning to visit the park on the 12th (Friday) or the 13th (Saturday). So I'm wondering what is the best date... Are the week-ends over-crowded? Should I have enough time to enjoy the park in one afternoon on Friday? Last thing... Could you tell me if it's the high season or if I should expect school groups?
Thanks for the help :)