According to this dataset on April 16th, 1907, there were 3702 immigrants processed. On April 18th of that same year, just 1314 immigrants were processed. So what happened on the 17th?
Here are all the ships that came in on the 17th of April, 1907:
Count | Date | Port | Ship
2409 04/17/1907 Naples Republic
2299 04/17/1907 Liverpool Carmania
2222 04/17/1907 Rotterdam, Holland Nieuw Amsterdam
1600 04/17/1907 Trieste Gerty
1317 04/17/1907 Liverpool Oceanic
521 04/17/1907 Copenhagen United States
491 04/17/1907 Queenstown Oceanic
401 04/17/1907 Christiansand United States
396 04/17/1907 Queenstown Carmania
375 04/17/1907 Christiania United States
232 04/17/1907 St. Michaels Republic
65 04/17/1907 Kr. Ania United States
22 04/17/1907 Kingston Prinz Eitel Friedrich
18 04/17/1907 Colon Prinz Eitel Friedrich
11 04/17/1907 Port Limon, Costa Rica Liberia
8 04/17/1907 St. Michaels, Azores Republic
8 04/17/1907 Savanilla Prinz Eitel Friedrich
5 04/17/1907 Jamaica Joseph J. Cuneo
2 04/17/1907 Cartagena, Colombia Liberia
2 04/17/1907 Savanilla Liberia
2 04/17/1907 La Guaira, Venezuela Maracaibo
1 04/17/1907 St. Lucia Pydna
Now compare that to the 16th:
1509 04/16/1907 Antwerp Finland
1480 04/16/1907 Havre La Gascogne
387 04/16/1907 Rotterdam, Holland Nieuw Amsterdam
163 04/16/1907 Havana Morro Castle
115 04/16/1907 Boulogne Nieuw Amsterdam
49 04/16/1907 Antwerp via Dover Finland
or the 18th:
1068 04/18/1907 Bremen Kronprinz Wilhelm
127 04/18/1907 Cherbourg, France Kronprinz Wilhelm
81 04/18/1907 Southampton Kronprinz Wilhelm
37 04/18/1907 Colon Allianca
1 04/18/1907 Baracoa, Cuba Fagertun
As is clearly evident, a lot of ships came in on the 17th. But not just any ships, in fact several of the largest ships of their particular fleets all arrived at Ellis Island that day. The Carmania (on the liverpool line) alone holds more passengers than the entirety that arrived on the following day on all ships. If you look further in the dataset, you can see that whenever the Carmania on the liverpool line pulls into Ellis Island, lots of passengers are discharged. Here's some more info on that ship in particular.
So I think on that particular 17th of April, a bunch of huge ships all arrived at the same time.