Is my full annual contribution to my Health Savings Account (HSA) available on the first day of the plan year (i.e., August 1) the same way it would be in the Health Care FSA?

No. You can use your HSA balance as funds are deposited, like a checking account. However, if you do not have enough funds to cover your health care expense, you may pay out of your pocket and reimburse yourself once your HSA funds have accumulated.