The IRS has announced this week that they will begin processing returns on January 28, 2019. So that will be the earliest date. However, it may take a couple of days for your return to actually process before it is either accepted or rejected by the IRS. See the TurboTax FAQ below for more explanation.
https://ttlc.intuit.com/replies/4193097
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***