Extreme ultraviolet (XUV) light source with attosecond (as) (1 as = 10-18 s) pulse duration enables new time-domain insight at the time scale of 10 - 100 as into the electron dynamics occurring in atoms or molecules. In particular, attosecond streaking of the electron photocreation and transport in the photoemission process has opened up a new perspective in the real-time electron transport. A strongly correlated electron system including d electrons could be often simplified as a local cluster due to their localized nature. This gives us a clue that attosecond physics of a strongly correlated electron system could be approached in analogy with atomic or molecular systems. In this study, we consider two subfemtosecond spectroscopic phenomena occurring in copper dihalides: (i) Photoelecctron emission timing for Cu 3s core lines, and (ii) Attosecond resonant photoemission and quantum interference. Copper dihalides are insulating solids of 3d transition-metal compounds with the strong electron correlation.