Hellebores (Lenten roses) start blooming in February in my Midwest garden, and often the winter aconites show their little yellow blooms in February too, accompanied by the dainty white snowdrops.
Before long wildflowers such as bloodroot, trillium, and Virginia bluebells appear, followed by other spring blooming bulbs including narcissus. Most spring bulbs will bloom in shade gardens under deciduous trees because in early spring they have not yet leafed out.
Later as the shade deepens under deciduous trees, shade lovers such as bleeding heart and brunnera, which is the perennial for-get-me not, and dainty little epimediums start their bloom cycle (in shade).
Shrubs such as clethra and hydrangea will also bloom in filtered light. Acanthus is another underused plant that grows well in partial shade. Of course, it takes up a lot of space but has fine architectural interest and handsome flower spikes.
Astilbes are stalwart bloomers in the shade, and the end of their cycle of bloom in my garden is provided by the lavender, low-growing Chinese astilbe that is a good edging plant.
In August and September, the perennial begonia ‘Evansia' provides a show-stopping display of pink blossoms. It's easy to have lots of these shade perennials and fill in any bare spots with the annual impatiens.
Note: Pulmonaria, hosta (unless you have deer), cranesbill geraniums, celandine poppy, and lady's mantle should also be considered for shade.