When Exactly Did Bob Welch Leave Fleetwood Mac?

[WalkAThinLine.]

In an interview with Goldmine magazine, Stevie said the following:

Mick called us right after Bob Welch left. He never said 'do you want to audition' or 'duo you want to come over and we'll get to know each other' or anything. Right from the beginning, it was 'do you want to join?'.

The article also says that the five musicians go together for the first time at a Mexican restaurant "a couple of days" after Mick asked the duo if they were interested in joining the band. This seems to suggest a late December departure for Welch, although as other users have mentioned above, I'm sure Welch was already contemplating his exit from the band before that.