Below were the planned phased release dates for 2020.1 (before any delays):
• Phase 1 - February 14, 2020
• Phase 2 - March 13, 2020
• Phase 3 - April 17, 2020
It'll likely be similar for 2021.1.
Each phase includes a subset of NetSuite Accounts. If you're on a shared instance then you are likely locked into whatever phase they assign to you. If you have a dedicated instance you can try to negotiate to move to an earlier or later phase.