The title points to Fat Tuesday, the last day of indulgence before Lent begins, and the poem's speaker is clearly standing on that threshold. The tone is blunt and a little sly. He opens with a bleak admission that the heart holds "only evil," then promises that tomorrow he will give up the urge. The deal is obvious: gratify the appetite now, renounce it later.
The short, clipped lines make the speaker sound casual, even flippant. "Late news is dead news" reads like a shrug, as if waiting is pointless and the moment is all that counts. Then comes the frank line about "a great lay tonight," with the final twist that tomorrow's discipline is the very reason for tonight's excess.
The mood is mischievous and self-aware. The speaker knows his logic is convenient, maybe a bit hypocritical, and he enjoys it anyway. Underneath the humor sits a sharper question about whether this kind of repentance is sincere or just a handy excuse.