cron man page — overview of cron.

crontab man pages in sections 1 and 5 — The man page in section 1 contains an overview of the crontab file. The man page in section 5 contains the format for the file and some example entries.

/usr/share/doc/at-<version>/timespec contains more detailed information about the times that can be specified for cron jobs.

at man page — description of at and batch and their command line options.