Reference: Regulation (EU) No 2017/373
When Regulations are published in the Official Journal of the European Union they invariably include several dates. The date of entry into force is usually expressed as the 20th day following publication of the Regulation in the Official Journal of the European Union. It means that the EU rules have been adopted and published - thus producing legal effects - but are not necessarily mandatory on the date of entry into force. The date when they become mandatory is the date of applicability.
The Regulation (EU) No 2017/373 entered into force on 21 March 2017 and the dates of applicability are specified in Article 10.