What do "sessions" and "summons" imply in Sonnet 30?
The language of business that appears later in the poem—the "expense" of things and people that the speaker has lost, a "sad account" that the speaker must "new pay as if not paid before"—brings to my mind some kind of a business transaction rather than a court. The speaker doesn't seem to be judging his memories so much as he is going over how much he has lost as he has aged and time has gone by: the loss of "waste[d]" time; "precious friends" that have died; "love" that didn't last or was, perhaps, unrequited; "grievances foregone"; and so forth. He considers his "losses," as he calls them in the final line, seeming to "summon" them and tally them up as one might when one is involved in a "session" or meeting for the purpose of doing business.
However, in the end, after calling the sad remembrances forth, he realizes that when he remembers his "dear friend" to whom the poem is addressed (using apostrophe: when the speaker addresses someone absent or dead or something which cannot respond), he no longer feels the great weight of his losses and cannot mourn them anymore.