This is reported on the front page of Kenya's official Electronic Travel Authorization (ETA) website (www.etakenya.go.ke): authorizations through the new method to enter Kenya and obtain a tourist visa will be accepted as of December 27, while still those who have made the "old" (evisa) online visa by December 31 can use that throughout 2024. So there is no risk that tourists will have to have both visas issued and that someone, arriving in early January, will be left without authorization.
However, the website itself recommends that the ETA application be made at least 72 hours before leaving.

We reproduce again the directions translated from the official website.

Who must obtain the ETA?

The ETA is mandatory for all foreign visitors, except for citizens of EAC member states who wish to travel to Kenya.

Every individual is required to apply for an ETA before beginning travel. It is mandatory to use the official government website to submit the application. Applications submitted through third-party websites will be automatically rejected. For those under the age of 18, the legal guardian, parent or accompanying adult is responsible for completing the application.

How do you apply for an ETA?

If you plan to travel to Kenya several times in the coming years, we encourage you to download the mobile application, which will allow you to store your data on your phone for reuse in the future.

The Government of Kenya has provided this website as a secure platform for visitors to submit information and apply for an ETA. ETA applications can be completed through this official website or through the official "Kenya Travel Authorization" mobile applications and must be made for each person who intends to travel to Kenya.

THE ETA IS ONLY VALID FOR ONE ENTRY INTO KENYA, ON SUBSEQUENT TRAVEL YOU WILL NEED TO REAPPLY FOR IT

Steps:

Visit www.etakenya.go.ke

Click "Apply Now" and follow the instructions

Make sure all data is entered correctly.

Submit the application

An e-mail will be sent confirming receipt of the application.

An e-mail will be sent confirming receipt of payment, if applicable.

After the immigration officer's decision, an e-mail confirming approval/rejection will be sent to you.

If your application has been approved, print the PDF document attached to the e-mail, download it to your mobile device or save it in the "Kenya Travel Authorization" mobile application. This is the travel authorization that must be shown at departure and arrival points.

If your application is rejected, you will not be able to travel to Kenya for the time being. Please contact support-kenya@govtas.com to report your situation and obtain further details.

You can check the status of your application at any time by visiting the following URL: https://www.etakenya.go.ke.

You will be able to travel only after your application has been approved by the Government of Kenya.