This forms large black tar-like spots on living Sycamore leaves.  In summer, apothecia contaning immature spores can be seen on the surface.  The spores mature over winter in fallen leaves, and in Spring the stroma cracks open and the spores are released in time to infect new leaves. .  

Photograph on leaf, note substrate and associated tree species

Mainly Sycamore, occasionally Norway Maple, rarely Field Maple 

Late Summer until the leaves fall in Autumn.

Very common