‘And she put off from her the garments of her widowhood, and covered herself with a veil, and wrapped herself, and sat in the gate of Enaim which is by the way to Timnah, for she saw that Shelah was grown up and she was not given to him to wife.'
When we consider Tamar's actions we must recognise that she has been grievously wronged. Basically she has been deserted. She had a right to marriage to Shelah, and the children that would result, because Judah had given her a promise. Shelah, of course, would not be limited to one wife. He had merely to fulfil his duty to Tamar and take her into his household and then he could proceed with his own life as he would.
So she takes the situation into her own hands. It may well be that if all else fails she has a right under the Levirate law to marry and have a child by Judah now that her mother-in-law is dead, for it is clear that the Levirate law reaches beyond just a brother (in the Book of Ruth Naomi is the wife whose husband has died, but Ruth expects to be able to bear children for her and uses the Levirate law to marry a ‘near-kinsman' of Naomi).
“She put off from her the garments of her widowhood.” Widows were expected to dress to demonstrate their status. We do not know of what this consisted but it seems that widows did not need to be veiled in public.
“And covered herself with a veil and wrapped herself.” She was not offering herself as a common prostitute but as a religious devotee. In many ancient religions a married woman would dedicate herself to the goddess of love, in this case Astarte, and would then be required to make love to a stranger, acting out that love. This may well have been a common practise to the Canaanites from whose background Judah's wife, and probably Tamar, had come. But it was repulsive to such as the family tribe of which Judah was a part (compare Deuteronomy 23:17). However, Tamar is only pretending to be available to strangers. She has only one person in mind. She wants to be impregnated with the seed of a near-kinsman of her husband as is her right.
“Sat in the gate of Enaim which is by the way to Timnah.” She sat where such women would commonly sit knowing that Judah must pass by that way to reach his destination. Compare Jeremiah 3:2; Ezekiel 16:25).