Some say goodnight — at night —

Some say goodnight — at night —
I say good night by day —
Good-bye — the Going utter me —
Good night, I still reply —

For parting, that is night,
And presence, simply dawn —
Itself, the purple on the hight
Denominated morn.
F586 (1863)  J1739

On first read it seems that Dickinson must have had her favorite people in mind when she wrote these stanzas. Surely there were those whose presence she wouldn't equate with "dawn". But maybe not. Her family was protective, indulgent, and comfortably well off, so Dickinson was under no pressure to entertain. She had a group of friends she was devoted to, but even those friendships were conducted largely through letters. Consequently, the presence of a friend was probably much more significant to Dickinson than to those of us who meet and mingle on a regular basis.
        Her circle of visitors became even more narrowed as time went on. Six years after writing this poem she wrote to her friend and "Preceptor" Thomas Higginson that she "should be very glad" to see him if he came to Amherst, but that she did not "cross my Father's ground to any House or town" (L330, 1869). She often did not even visit with dear friends who came to see her.
        Her aversion to saying "Good-bye" or perhaps to parting in general might be explained by the intensity of her feelings. When one of her dearest friends, Samuel Bowles, returned from a seven-month trip to Europe in 1862, Dickinson stayed upstairs during his visit, sending only a note: "I cannot see you. You will not less believe me. That you return to us alive, is better than a Summer. And to hear your voice below, than News of any Bird" (Cynthia Griffin Wolff, Emily Dickinson, p. 399). In the late 1870s when beloved friend and sister-in-law Susan Dickinson returned from a two-week vacation, the poet wrote, 'I cannot see you for a few days. You are too momentous. But remember it is idolatry and not indifference" (Hart and Smith, Open Me Carefully, p.220).

One reason Dickinson felt the presence of friends and family so powerfully is her dread of their death. 

I should not dare to leave my friend,
Because – because if he should die
While I was gone – and I – too late – 
 …My Heart would wish it broke before – 
(F234)

She writes this concern more explicitly in a letter written in 1852: "I look at my father and mother and Vinnie, and all my friends, and I say – no, cant leave them, what if they die when I’m gone” (L86 to Jane Humphrey).

With all this in mind, is it any wonder that Dickinson equates parting with night – itself symbolic with death? And so she chooses to always say "Good night" when parting with company, even during the day. It's almost comical, and she seems to recognize this. The first stanza makes light of this quirk with the repetitions of "Good", "Good night", and "night". She hammers the "g" alliterations with a "Going". We can just imagine the departing friend shaking his or her head in a sort of "that's Emily for you" way.

Dawn sky, vgamenut 

        The second stanza ends the poem with a positive affirmation of what the "presence" of her company means. It is "simply dawn" and dawn is the purple crowning of morning. This almost rapturous response to the presence of a friend or dear one must have taken a toll.
Some scholars have suggested that one reason for the period of estrangement between Emily and Susan Dickinson was Susan's exhaustion with Emily's intensity. Further, Dickinson's friend and poetry "Preceptor" Thomas Higginson wrote his wife 1870, that “I never was with any one who drained my nerve power so much. Without touching her, she drew from me. I am glad not to live near her.” (Brenda Wineapple, White Heat, p.180). 

What a remarkable friend she must have been!