When harvesting flowers for everlastings, I check the dark flower heads daily and collect them only at the swollen bud stage, when just one or two of the flowers have opened on the flower head. Picking the flower heads at this time preserves their dark color and ensures that the flowers will stay attached to the stalk once dried. Picking flower heads once they have passed this stage results in paler flower colors and shattered flower heads. When harvesting lavender for potpourri, I'm less concerned about shattered flower heads and flower colors than I am when collecting flowers for drying on the stalk to make everlastings. I don't harvest white or pink flowering varieties for drying on the stalk, as their colors turn muddy when dried. However, they are fine for potpourri or sachet making. L. angustifolia and its cultivars are the best lavenders to use for potpourri because the oil quality is the best.
Here are the varieties of lavender you can pick this weekend.
Buena Vista
Hidcote
Royal Velvet
Purple Bouquet
Maillette