So many people have been justifiably disappointed lately because tickets are sold out for their dates. I learned from their misfortune and hope what I write helps others.
First, I started stalking tickets for various dates a week a head of time.
Tickets go on sale at midnight Paris time, 6 pm East Coast US, 3 pm West Coast.
I noticed that by the time I would wake up in the morning Paris time, six or seven hours after sales began, that all time slots were sold out except 11:00 pm.
So, I knew I had to get the tickets at midnight French time. It's for a special occasion, so I couldn't fail the family.
By 11:55 pm I had everything filled in: number of adult tickets, number of children's tickets, floor, elevator...
At midnight I refreshed my computer and our chosen date two months in advance appeared for the first time.
I used a full keyboard for 10-finger typing because you have to fill in the names for each ticket. Then my husband read the credit card info so I wouldn't have to stop and look. I had to hit "valid" several times before it would go through.
By 12:02 we had the tickets.
Note: some time slots were already sold out at midnight at the first minute of general sales! But remember that the tower, a privately run entity, has separate sales for "professionals" and "individuals." So this is one example of where the sold-out times may have already gone before they went on sale to the general public. https://www.getyourguide.com/eiffel-tower-l2600/ Perhaps overseas tour groups have access to these tickets first, too? IDK.
But, when people say to keep checking back as the time gets closer, that makes sense. I'm sure the above tours and many others turn tickets back in if they haven't been resold for a tour. The one and only time I have gone up the tower was after a heavy rain. All the tour groups had cancelled, the sky was now clean and clear just as the sun was setting, and we zipped right up on the elevator. So keep looking for tickets, or pray for rain that keeps other away. Good luck.