It’s best to plan to arrive two or more days before starting the trek.
This gives you valuable time to rest, recover from the journey, start the acclimitization process (Arusha is over 1,400 elevation), and have time for getting your gear in order and get a detailed briefing and gear check. We recommend taking a day trip to Arusha National Park, or a short safari before your Kili trek. The more days you are in Tanzania before Kili are going to be an asset, to get you more rest and recovery and get you acclimatized to higher altitudes.