Dropbox eligibility end date


My H1B visa expired in August 2019. The general drop box application eligibility for expired visa’s is 12 months but due to COVID they increased the eligibility to 24 months. This means in general case, I do not qualify for drop box but due to special COVID exception I am eligible to avail this option right now. This exception is valid till March 31st 2021 currently.
What exactly does this drop box March 31st deadline mean:

I can schedule H1B visa drop box appointment till March 31st for say April or May month OR I need to do actually drop off my documents before March 31st?