A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
gen 38:1JUDAH AND FAMILY. (Gen. 38:1-30)
at that time--a formula frequently used by the sacred writers, not to describe any precise period, but an interval near about it.
gen 38:2And Judah saw there a daughter of a certain Canaanite--Like Esau [Gen 26:34], this son of Jacob, casting off the restraints of religion, married into a Canaanite family; and it is not surprising that the family which sprang from such an unsuitable connection should be infamous for bold and unblushing wickedness.
gen 38:8Judah said unto Onan . . . marry her, and raise up seed to thy brother--The first instance of a custom, which was afterwards incorporated among the laws of Moses, that when a husband died leaving a widow, his brother next of age was to marry her, and the issue, if any, was to be served heir to the deceased (compare Deu 25:5).
gen 38:12Judah . . . went up unto his sheep-shearers--This season, which occurs in Palestine towards the end of March, was spent in more than usual hilarity, and the wealthiest masters invited their friends, as well as treated their servants, to sumptuous entertainments. Accordingly, it is said, Judah was accompanied by his friend Hirah.
Timnath--in the mountains of Judah.
gen 38:18signet, &c.--Bracelets, including armlets, were worn by men as well as women among the Hebrews. But the Hebrew word here rendered "bracelets," is everywhere else translated "lace" or "ribbon"; so that as the signet alone was probably more than an equivalent for the kid, it is not easy to conjecture why the other things were given in addition, except by supposing the perforated seal was attached by a ribbon to the staff.
gen 38:24Bring her forth, and let her be burnt--In patriarchal times fathers seem to have possessed the power of life and death over the members of their families. The crime of adultery was anciently punished in many places by burning (Lev 21:9; Jdg 15:6; Jer 29:22). This chapter contains details, which probably would never have obtained a place in the inspired record, had it not been to exhibit the full links of the chain that connects the genealogy of the Saviour with Abraham; and in the disreputable character of the ancestry who figure in this passage, we have a remarkable proof that "He made himself of no reputation" [Phi 2:7].