Genesis 17:23

‘And Abraham took Ishmael his son, and all who were born in his house, and all who were bought with his wealth, every male among the men of Abraham's house, and circumcised the flesh of their foreskin on the selfsame day as God had said to him.'

Abraham obeyed immediately. No doubt he gathered the men together and explained the wonderful theophany he had experienced and described the terms of the covenant, and then the ceremony would take place as a dedication to the God of the covenant. But what matters is that the demands were fulfilled. Then the covenant was put into writing and the following final verses are the confirmation of the fulfilment of the demands of the covenant.