The letters

Every day, another letter flew through the letter box. Every day they got increasingly worried. Every day they concealed themselves no they could at least try to remain safe.
"10 days"
"9 days"
"8 days"
"7 days"
"6 days"
"5 days"
"4 days"
"3 days"
"2 days"
"1 day"
" Time's up!"