Why we use 150 mm size cube for compressive strength testing

Concrete Cube Testing & NDT concrete cube dimensions For cube test two types of specimens either cubes of 15 cm X 15 cm X 15 cm or 10cm X 10 cm x 10 cm depending upon the size of aggregate are used For most of cube test procedure pdf The mould size should be cm cube as per IS 4031 1988 flag Suggest Corrections

cube test report size concrete cubes tested at 28 days Why do we test at 7, 14 & 28 days? Concrete is a macro content with Sand, Cement, & Coarse aggregate as its micro