The title sets up a contrast, and the poem splits neatly along it. The first two lines treat joy as an invitation, almost an order: you are called to the table, and what's served is rich ("fat"). Joy here isn't delicate. It's abundant, bodily, something you accept by showing up hungry.
Then the speaker turns to fear. The "children of discontent" are people who are never satisfied, and they're pictured working late, "setting papers in order." It's a quiet, almost harmless image, yet the speaker finds it unsettling. Their busyness looks like a way of keeping life tidy and under control, while joy goes unanswered.
The last line, "night new-laced with night," makes the dark feel layered and deepening. The mood is uneasy and a little cold, and it sits in sharp contrast to the warmth of the opening feast. The poem seems to ask which life you'd rather be called to.