Ĵaŭde is the form of Ĵaŭdo that signifies something done repeatedly, every week on that day. I drink beer every Thursday - Ĵaŭde mi trinkas bieron.

I believe this is because after prepositions, such as "en", the accusative case (the -n) is not used. See the Notes at the start of the lesson on the accusative: "The noun following a preposition normally takes a simple -o ending (-oj in the plural). Other endings will be explained later. For example: La virino estas en la kafejo. The woman is in the café."