An A/B test can be scheduled for later. Just like you schedule a newsletter for later.
Make your A/B test so that you have an A Variant and a B Variant.
In the summary, you can see again what you have entered:
If you choose 'immediately' then your A/B test will be started immediately.
When you choose 'send at' you can enter a date and time when you want your A/B test to start.