This rather depends on the particular bulbs. As a quick rule of thumb, Spring-flowering bulbs such as tulips, daffs etc are all planted in the autumn (Oct-Nov) whereas those that flower in the summer such as lilies, gladies and such will be planted March -Apr.
Some bulbs are best planted after flowering while they still have leaves on. Snowdrop, muscari and bluebell are a few. There is usually only a few weeks in the year when these are available 'in the green' as they quickly go dormant.