Thursday 13 Mar 2008 Information 
This is sample text
Friday 14 Mar 2008 Warning
This is one more sample text

#!/bin/sh

date=
input=
nl="
"  # yes, that's opening quote, newline, closing quote
while read line; do
  case $line in
    *day\ [0-9][0-9]\ *\ [12][0-9][0-9][0-9]\ *)
      # warning! y3k problem
      # warning! fragile matching, would be better with regex
      # warning! basically untested code anyway
      case $date in '') ;;
        *) echo "Result for '$date'":
	    echo "$input" ;;
      esac
      input=
      date=$line
      ;;
    *)
      input="$input${input:+$nl}$line"
      ;;
  esac
done

# whatever is left at end of file is a result too
echo "Result for '$date'":
echo "$input"