#!/bin/bash

cd $(mktemp -d)

echo ">day_1
ABC
DEF
GHI
>day_2
JKL
MNO
PQR
>day_3
STU
VWX
YZA
>month_1
BCD
EFG
HIJ
>month_2
KLM
NOP
QRS" > input.txt

lines_by_subcategory=3 # number of lines *after* a subcategory's header
for category in 'month' 'day'; do
    grep ">$category" -A $lines_by_subcategory input.txt >> "$category.txt"
done

echo "directory content :"
ls

echo "month.txt :"
cat month.txt

echo "day.txt :"
cat day.txt