For the early morning Tiruvaradhanam during the month of Margazhi, at what time must we gather flowers, Tulasi, etc.? Can they be plucked before sunrise on the same day, or must they be gathered the previous day?

Tulasi can be plucked the previous morning; Tulasi must not be plucked in the afternoon or before sunrise.
Full blown flowers can be gathered; at times flowers are full blown around 5 AM, before sunrise and such flowers can be gathered during that period.