Plan it before Stage 1. At Stage 1 the certification auditor reviews your documents and checks whether an internal audit and a management review are planned and have been performed; the rules for certification bodies (ISO/IEC 17021-1) put it exactly that way. A missing internal audit is one of the most common Stage 1 findings, and it can delay Stage 2.
Some bodies accept an internal audit scheduled between the two stages. It then has to be complete, with findings being closed, by the time Stage 2 starts, and the gap between the stages is often only four to six weeks.
Doing it before Stage 1 gives you a cleaner Stage 1 and time to fix what it finds. A workable timeline for a Stage 1 date in June: internal audit in mid-April, report by the end of April, management review in May with the findings on the agenda, and the first corrective actions closed by the time the Stage 1 auditor arrives.