Replace "weekend" with "weakened" and it should make sense. It's a play on words since normally you would say you have a weakened immune system if you get sick a lot, but since they're only well on weekends, it's a weekend immune system.
I played a knock-off flash game called "Balloon Duel" as a kid. It was pretty fun, so I just decided to buy Balloon Fight to try it out. Thanks for the recommendation!
For a second I was like "Yo, is that logo a cloudberry?" Then I looked it up and apparently lakka is Finnish for cloudberry. Makes sense, haha!