for those who say "the baby can't be in a routine at this age," of course it can. Now, this might not mean that the baby is in its own routine where it always sleeps from x o'clock until y o'clock, never waking unless it is in a certain environment, but it is perfectly possible, if the op has a baby who settles well in certain circumstances e.g. in a crib in a darkened room, that settling the baby in this way every night ensures that A, the op has some more op time in the evenings, and B, the baby gets some decent sleep and is not unsettled later at night.
There are plenty of people on mn who say that "my baby will only sleep if he is being cuddled/held/driven down the m25," and nobody argues with those people, they tell them to just do what it takes if it means the baby sleeps. So why are people whose babies sleep better in a quiet unstimulated environment expected to change their expectations/routines to accommodate others?
And if being out means the baby is more unsettled at night it's not the mil who is going to be dealing with it in the middle of the night.
"it's only one night," not if the op is up every other night doing night feeds so is sleep deprived anyway, and this means that she's going to be getting even less sleep than she usually does.
Nfw would I have taken my baby out at that age, or passed him around to all and sundry. Anyone who felt the need to criticise me for that would have been free to do so.
There are lots of things which can be construed as gently pfb. Not wanting to go out with a three week old baby and then expecting it to be passed around a dinner table and to then go home to a 4 AM screaming session is not one of them.
People who have these expectations clearly are actually not thinking about the baby or the op, but themselves.