Thommo has plenty of happy memories from his appearances at the National Stadium with Rangers and Scotland, but says sending Saints into the Scottish Communities League Cup final would top the lot.
The Paisley men have now lost eight matches on the bounce against the Hoops under boss Danny Lennon, conceding 22 goals without reply.
The Buddies' winless run against Celts stretches back almost 23 years.
Thommo said: "It is important for us that we go to Hampden and give it our best shot and leave with no regrets.
"Yet at the same time we have got to watch and not go too gung-ho, so it is going to be about getting the balance right.
"The last time we played Celtic we adopted a pragmatic approach and still lost out 2-0 at Parkhead in December.
"I would hope that we will be far more positive in our approach this time. It is a cup semi-final and we know that we will take a big support to Hampden and we want to give them something to cheer for.
"I know all about the statistics and the fact we haven't beaten Celtic in eight games under the gaffer, or scored against them for that matter, but it is about what happens after eight that counts.
"We know on Sunday we will be the underdogs and hopefully we can finally have some joy."
He added: "Sunday will be a huge moment for me and to get the chance to lead the line for my hometown team in a League Cup semi-final against Celtic is something I never thought I'd experience. I am determined to make the most of it."
Looking back over his illustrious career, Thompson has no doubt about his Hampden highlight.
He recalled: "That was definitely the League Cup semi-final, back in 2005, when I was playing for Rangers.
"We played Dundee United, whom I had signed from for Rangers. Although I was on the bench, I got on for Dado Prso with 15 minutes left to go and score twice in a 7-1 win.
"I would settle for a repeat of that performance on Sunday."
While Thommo admits that the Saints' 4-1 SPL defeat at the hands of Ross County last time out was far from ideal preparation, he is confident that his side can bounce back.
The 33-year-old said: "I don't think the County defeat will have any bearing on the Celtic game.
"This is a one-off at Hampden and I would say the ideal game to have next up after a defeat like County.
"We have boys in our squad who have never played at Hampden and the guys like me, who thought the chance to play at the National Stadium had gone.
"At St Mirren Park there are old photos of previous cup winning sides and we want to be up there alongside them.
"But we'll have to do it the hard way."