we use then in this way to mean "at that time" or at the specific time we have previously spoke about. the sentence should say, "i will look for work in (the months of) june and august because my university vacation is then" i put "in the months of" in brackets because its not relaly needed. so the sentence has already stated juny and august as the time of reference, so "then" is rfering to juny and august. you can use it in the same way in other sentences. for example: A- i need to play my piano but i dont like playing it when other people are home B- oh its ok we are going out soon A- oh, i'll play it then then. this may seem funny because i wrote then twice, but this is exactly how a spoken conversation would take place. though it might not be gramatically correct. btu as you can see "soon" is the time that "then" is refering to.

Then in this car = at that time. I look forward to "then- the time of" the months June and August.

I usually look for work during the summer months because my university vacation is at that time, (then), or because my university vacation is then.