There's a couple variables and deciding when to see the comet. So, there are multiple "best days," but this video details all the variables you need to plan to see it.
At the end of September / beginning of October, it will become visible and large in the morning, mid-october it switches to an evening comet but is in the sky with a full moon or is close to the horizon, by mid-late-October, it's higher in the skies with no moon and potentially a big tail but dimmer.