A rendezvous began as an order, not an appointment.
Rendezvous is a French sentence turned into a noun. Etymonline says it comes from French rendez-vous, a noun use of rendez vous, which it glosses as "present yourselves." The phrase was taken from the wording of orders, which suggests a military origin.
The grammar is simple. Rendez is the second-person plural imperative of rendre, "to present," and vous is "you." Etymonline traces vous to Latin vos. Merriam-Webster agrees that both the noun and the verb come from Middle French.
Etymonline dates the English noun to the 1590s, with the first sense "place appointed for assembling of troops." The general sense of an appointed meeting place is also attested in that decade. From about 1600 it meant a meeting held by appointment.
Merriam-Webster gives an earlier date. It lists the noun from 1556, in the sense of a meeting at an appointed place and time. The two dictionaries disagree, so the safest claim is that English had the word by the second half of the 16th century.
The verb is later in both. Etymonline has it in the 1640s, meaning "assemble at a particular place," derived from the noun. Merriam-Webster has the verb from 1645.
The root verb is older than French. Etymonline says Old French rendre meant "give back, present, yield" in the 10th century. It came by way of Medieval and Vulgar Latin rendere, a variant of Latin reddere, "give back, return, restore."
Reddere joins red-, "back," to a form of dare, "to give." Etymonline suggests the Vulgar Latin change to rendere was either simple nasalization or an analogy with prendre, "to take."
So a rendezvous is literally a command to present yourselves at a place, as ordered.