internal audit plan vs. internal audit program

And internal audit program is defined in various standards (like ISO 19011) as "one or many audits". An audit plan is generally speaking the audit activities of one audit, including timing/duration/responsibilities etc.

Most people use a yearly calendar of audits, but I'd counsel against that because it's a) not required, b) will lead to ineffective audits being done in areas of the business when there's no need/too late/early etc and finally c) you'll be updating it regularly (and if you aren't you should be!)

I use a moving 30 day "window" and look at past recent issues (product problems, customer feedback etc) or (as the ISO 9001 9.2.2 requirements state) changes and importance of processes. These kinds of things don't happen (usually, except in a few cases) to a rigorous calendar.