First mosaic, all at once or per night?

I am also created my first mosaic over multiple days.  I do all panels each night so the seeing and the gradient is distributed equally.

Gonna pile on to the all-in-one-night crowd because you want consistent seeing and gradients in all your frames. This becomes more and more important the closer your target is to the horizon. You also need to strike a balance between efficient use of time and how little variation you want between panels. You could, for example spend one hour per panel before switching, but that might be too long given a particular scenario. The flip side is that you bounce back and forth every 10 minutes, but that is a lot of time lost to slewing, plate solving, and focusing again.

Generally speaking, the closer your target is to the horizon the less time you want to spend on each panel before switching.

....and if going more than one night you might want to reverse the order of the panels, this is to even out the effects of altitude.

What do ya mean by this?? I haven’t done a mosaic just yet, m31 or the veil nebula will be
my first