Some of the days are also marked in blue or purple.

I looked at the timeline for events that Lostpedia.com has and didn't see anything consistent with the different colors used to x out days on the calendar. For example the days marked with yellow x's are Oct 20, 21, 22, and 23. Here are the events that took place on those days.

Day 29 - Wednesday, October 20, 2004
"Homecoming"
Scott is found murdered on the beach.
Ethan is lured into an ambush and is captured. Charlie shoots him four times, killing him.

"Outlaws"
Sawyer hunts a boar that ransacked his tent. He hears whispers in the jungle repeating Duckett's last words.

Day 30 - Thursday, October 21, 2004
"Outlaws"
Sawyer tells Sayid he heard whispers.
Charlie and Hurley bury Ethan.
Kate offers to help Sawyer track down his boar in exchange for access to his stash. At night, they play "I never".

Day 31 - Friday, October 22, 2004
"Outlaws"
Sawyer confronts the boar but lets it live.
Sawyer returns the gun to Jack, and realises that he met Jack's father in a Sydney bar.

Day 32 - Saturday, October 23, 2004
"Exposé"
Arzt, Shannon and Kate argue about the guns from the Marshal's case.
Paulo retrieves the diamonds but doesn't tell Nikki.

"...In Translation"
Walt sets fire to the raft as he does not wish to leave the Island.

__________________

Doctor playing golf. Woo, boy howdy, now I've heard everything. What's next, cop eating a donut?