There are different views on the 'best day'. In Canada, the general advice is that the best day is Tuesday or Wednesday because it's in the middle of the work week. If you think of the person being let go: they can go home and start talking to others regarding their new situation. Maybe they predicted their fate and had a resume ready that can be sent right away to recruiters. They can contact HR during that week about their benefits or other questions. They can contact their lawyer as well. If you arranged outplacement support, they can start using those services right away.

In contrast, if you let them go on a Thursday or Friday, you ruin their weekend plans and their perception about your company will be much worse. But of course, it's a very difficult situation regardless of the day you choose. Thanks for sharing your experience!