Emily Dickinson refers to the color purple in many of her poems ("There is a flower that bees prefer", "Wait Till the Majesty of Death", and "We like March" to name a few). Again, the poem "It will be Summer--Eventually" (Poem 342 from The Complete Poems of Emily Dickinson) is no different.
The Lilacs—bending many a year—
Will sway with purple load—
As seen in the use of purple in Dickinson's other poems, the use of the color purple typically refers to the fact that
purple has a special, almost sacred place in nature: lavender, orchid, lilac, and violet flowers are often delicate and considered precious.
Purple is the color of good judgment. It is the color of people seeking spiritual fulfillment. It is said if you surround yourself with purple you will have peace of mind. Purple is a good color to use in meditation.
This being said, the use of the color purple in this poem denotes the color which Dickinson most aligns with spring and summer.
The time of the poem is during spring, given summer has not come yet- "it will be summer-eventually." Dickinson spends the nineteen lines of the poem detailing what is going to happen once summer comes.
The exact meaning of the line "will sway with purple load" refers to the lilacs in the previous line and how they will be heavy with both color and petals once summer erupts over them. The purple refers to the color of the lilac flower's petals.
The poem is simply Dickinson's thoughts on what happens once summer comes- women will bring out their parasols, men with canes, and children with their dolls will be seen on the streets.
All of nature will repeat the "lives" which they have lived season after season. When summer comes to an end- "folds her miracle"- the women will put away their summer dresses, the priests will adjust symbols after the sacrament (religious ceremonies showing Divine grace) as to show the promise of another spring and summer to come.