I've just come across Wikipedia's page about Athens Cathedral, which seems to suggest - but I may be giving an incorrect interpretation of its meaning - that the church may be currently closed.
("...Currently, the Cathedral is going under renovation. Archbishop Ieronymos of Athens announced at the beginning of 2009 that the Cathedral will be closed for a year due to a revamp.")
I recall, though, very recent threads on the forum where locals suggested attending next Easter Mass at the Mitropolis, so I assume that the church is open and operating normally. Is that correct?