is the accusative used after an infinitive?

The tense or form of the verb doesn't matter. If the verb is transitive, the direct object should be in the accusative.

Having said that the Esperanto expression fari tagon literally means, that you fabricate or produce an object such as a day, as if you were a god making the day to rise. The English expression "to make a day" is an idiom with a different meaning: to make someone happy or be a source of satisfaction, for the rest of the day. To make someone happy is in Esperanto feliĉigi:

• Bonaj infanoj gepatrojn feliĉigas (by Zamenhof)