According to the USCIS:
"The initial registration period for the fiscal year 2021 H-1B numerical allocations will close at noon Eastern on March 20. Registrations must be properly submitted before noon Eastern on March 20 to be considered in the initial selection process.
If USCIS receives enough registrations during the initial registration period, we will randomly select the number of registrations we project are needed to reach the FY 2021 H-1B numerical allocations and send selection notifications via users’ online accounts. We intend to notify registrants with selected registrations from the initial registration period no later than March 31, 2020, that they are eligible to file an H-1B cap-subject petition for the beneficiary named in the applicable selected registration. An H-1B cap-subject petition must be properly filed within the filing period indicated on the relevant selection notice. The period for filing the H-1B cap subject petition will be at least 90 days."