Day suffered the initial knock in Saints' 24-6 victory over London Wasps in the Aviva Premiership back in September, and required further surgery after repeating the injury upon his return.
But preparing to return this weekend in the LV= Cup after a week of training, the 29-year-old insists it is crystal clear what he now has to do in order to win back his first-team place.
And after a strenuous and rather tedious period of rehabilitation, Day believes he has a great opportunity to prove what he can do with his side's international stars now absent.
"For any sportsman, if they get injured they want a time frame to come back in, but to injure the same thing again and to have done it immediately on their return is as frustrating as it gets," said Day.
"I have gone through a long period of rehab which is quite mind-numbing but my ankle feels stronger and I have put on a few pounds in the gym so I am ready to get out there.
"I trained last week with the team but wasn't selected for the weekend but I am in the frame this time.
"It is ideal that there is at least one space to fill. Courtney [Lawes] will be away with England, but it won't be easy to get back into that team as people have stepped up.
"I just have to take my chance when I am handed it and the LV= Cup, combined with the internationals, is a great opportunity."
Looking to put their recent Heineken Cup exit at the hands of Glasgow Warriors behind them, the Saints will take on Gloucester Rugby in the LV= Cup in what, according to Day, is a strange period of games for most clubs.
And with the likes of Lawes away on international duty and Calum Clark out injured, Day is looking to seize his chance along with some of the younger squad members.
"It is the start of a very strange period for a lot of clubs. A lot of the bigger personalities disappear to the internationals and the fringe players step up," he added.
"We were under no illusions going there [Glasgow] that we would get the five points and we are disappointed but there is no doom and gloom in the camp.
"We are looking forward to playing in the LV= Cup and give some of the guys a chance. They are tough games and they need to step up whilst others are away.
"We have always been a team that treat the LV= Cup seriously. We put out competitive sides and have done well in the competition in the last few years.
"This is the chance our young guys have been waiting for, a chance for them to show what they can do.
"We are well placed in the group and even though Gloucester are likely to have a lot of changes they will still pose a big threat."
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