This time next week, we celebrated our parents 25th wedding anniversary with our relatives in our new house.

Explanation-As given in the question 'the time next week' represents future time action so verb celebrated is wrongly used here. The given time is in future continuous tense and in future continuous tense we use Will/shall + be + v1+ing, as we know will and shall represent the probable action for the future time and they are also modals. So, with modals we use base form of the verb so we use ‘Be’ with will and shall and to make it continuous we use V1+ing. But when the subject is 'WE' then we use shall with 'WE' Therefore, Shall be celebrating is correct. So option (C) is correct.