According to my journal, last year we started cutting lavender around May 17. Whether you have one lavender plant, or hundreds, the question of when to cut depends partly on how you intend to use your lavender. For a fresh bouquet, it's usually recommended to cut the spikes with the flowers in bud, or when a quarter to one-third of the flowers on the spike are open. Lavender that is to be dried can be cut when the first two flowers on the spike have opened. If most of the buds have opened, the flowers will fall off when dried...in which case they can still be used for potpourri or sachets. Harvesting lavender for its essential oil depends on the variety - frequently after the flowers have opened and started to dry out.
Choose a dry, sunny morning, after the dew is gone, and use a good, sharp pair of clippers or scissors. Cut 6 to 8 inches below the blooms, or just above the woody part. For dried lavender, secure the stems in place using a rubber band or tie together with yarn or raffia. Hang upside down to dry in a cool spot away from sunlight. Fresh lavender stems should be placed in water. Remove any foliage beneath the water line, and change the water daily to keep it fresh-looking. Then, sit back and enjoy the smell!