To be held from September 17 to February 13; park fully reopened from June 26
— ユニバーサル・スタジオ・ジャパン公式 (@USJ_Official) June 27, 2021
The theme park closed temporarily with the declaration of the new state of emergency on April 25 due to the new coronavirus disease (COVID-19). It tentatively reopened on weekdays only starting from June 1, and it fully reopened from June 26.
Source: Universal Studios Japan website