Late Thursday evening or early Friday morning.
Fridays are a busy day in Post offices with pensioners.
Although you can leave it a couple of weeks and collect in one go too.
Just click on the link and the URL anchor brings you right to it...
Payment Day
A Widow's, Widower's or Surviving Civil Partner's Contributory Pension is paid on Friday, weekly in advance. See “Payment Related Issues” for more information.