Windows are currently open on different dates in different countries.
European football's governing body has asked its member associations to adopt the "harmonised" date, with a player registration deadline on 6 October.
Uefa's executive committee also announced emergency measures to provide clubs with financial flexibility because of the coronavirus pandemic.
It wants to help clubs "neutralise the adverse impact of the pandemic".
In a statement, Uefa said the measures aim to:
Give clubs more time to quantify and account for unanticipated loss of revenues
Allow clubs to adjust the break-even calculation for revenue shortfalls reported in 2020 and 2021
Ensure equal treatment of clubs where the impact of coronavirus may be realised in different reporting periods
Address revenue shortfall
Retain the spirit and intent of financial fair play for football's long-term viability
Uefa also announced the appointment of new members to its disciplinary bodies "with a view to allow for more diversity in the composition of these bodies".
Former West Ham, Northampton and Peterborough footballer Bobby Barnes has been appointed to the control, ethics and disciplinary body.