Go-live is a guess until your busiest staff run a full day in the new system
If your two busiest employees have not run a full working day in the new system before go-live, the go-live date is a guess.
A common pattern: the new system goes live on Monday. By Wednesday the staff are back on the spreadsheet, quietly entering everything twice so nothing gets lost. By the end of the month the owner is paying for software the business routes around.
It can be caught weeks earlier. Usually training happened in a one-hour session, run by the vendor, on the vendor's sample data. The people who do the work every day were consulted last, and their first real Monday morning in the new system was an actual Monday morning.
The fix is a dress rehearsal. Pick the two people who handle the most volume. Give them a full day in a test copy of the new system, working through last week's real orders and customers, with the usual interruptions, before the cutover date is final.
Write down every time they reach for the old way. Each one is a missing setting, a training gap or a sign the workflow does not fit, and all three are cheaper to find before the switch.
If the vendor resists a rehearsal, ask what proof of readiness they would accept instead, and put that test in the plan.