60 days in advance.
"The desired visit date is not available, what should I do?
montrer/masquer la réponse
Certain days (weekends, days during peak season between July and August) are in high demand and we sell out quickly, especially for e-tickets for the top floor, which are the most popular. If you are flexible, select another date that is shown as available. The dates marked in orange indicate that the last tickets are available. Lined out or grayed out dates are no longer available. If there is no more online availability, note that tickets to visit the Eiffel Tower are also sold on site at the ticket offices at the monument.
Regardless, we sell our tickets online up to 60 days in advance (for lift tickets) and 14 days in advance (for stairs tickets to second floor). If you can, plan your visit well in advance, so you will have the choice of date, destination and time."
https://ticket.toureiffel.paris/en#_ga=2.35289321.915584354.1712851877-1515458336.1712851877