The language of many of the characters in this book is difficult, because they speak in a dialect which is not entirely real (it is a fictionalized Yorkshire / Northern English).
What the speaker means here is: "Now that the dairymaid knows that she's going to be married by Michaelmas, she's as likely to pour the new milk in the pig trough as into the pans."
Michaelmas is is a Christian holiday celebrated in September. The dairy maid is excited about her marriage and the speaker expects her to not perform well at her duties because she is distracted by her excitement. She might, for example, mistakenly feed the fresh milk to the pigs instead of saving it for the household.
This is not standard English speech you will hear in modern SAE or RP.