(Original Publication Date, 23 September 1990)
Panel 1: We find ourselves watching Elly on the phone having a conversation with an unknown party. For some reason, whatever this person did with a bedpan is the height of hilarity. This tells me that Mystery Person is in the hospital.
Panel 2: Now that the conversation is nearing its end, she smiles as she tells Whoever to keep smiling and take care of him/herself before being asked a question.
Panel 3: Said question appears to be "Will you be visiting soon?" and the answer is "Yes."
Panel 4: Now that the conversation is over, Sundayverse Elly is seen to weep openly at someone else's problems because Sundayverse Elly is only mildly self-absorbed. Sundayverse Liz asks her what's wrong.
Panel 5: Sundayverse Elly explains that she has a good friend who's dying. When he calls up, he makes her laugh.
Panel 6: However, when the call is over, the sadness of his situation overwhelms her.
Panel 7: Elizabeth explains this as being what she refers to as a rainbow day. Elly murmurs a bit in confusion because Sundayverse Elly doesn't freak out so much.
Panel 8: Liz panders to the churchgoers by saying that when the sun is shining when it rains, God makes a rainbow.
Panel 9: This means that when one laughs and cries, one is having a rainbow day.
Panel 10: Having been charmed by the analogy, Elly says that Liz is a neat kid. Liz replies by saying she loves her.
Summary: This, I think, is what people mean when they say they love the strip: it's what Lynn is capable of when she doesn't succumb to her belief that pointless acrimony is what people want to see. Sure, it's manipulative but at least people aren't foaming at the mouth with rage over nothing or whining because things don't go their way.
Also, howtheduck is right to say that this is a farewell to her father because the chronology lines up really good.