Variable: package-enable-at-startup

Whether to activate installed packages when Emacs starts.
If non-nil, packages are activated after reading the init file
and before `after-init-hook'. Activation is not done if
`user-init-file' is nil (e.g. Emacs was started with "-q").

Even if the value is nil, you can type M-x package-initialize to
activate the package system at any time.