The original effective date of the MDR was May 26 2020 but has been postponed until 26th of May 2021. As of this date, the new rules apply immediately for class I devices, devices that end up in a higher class under the MDR than under the previous directive, and for new devices. Class I devices that end up in a higher class under the MDR and devices that are already on the market and still have a valid MDD certificate are excepted from this date. Under certain conditions, these devices can still be marketed for a while according to the old directive, the MDD.
The "When MDR?" quick scan will help determine when your medical device should comply with the new rules.