Bash read parameters from file

If you have control over this file, a much better format would be one number per line, or perhaps a whitespace-separated line of just the numbers.

If not, you can preprocess this wicked format into something which is easier to process with shell script with a simple Awk snippet.

awk '/^Days: / {
  n = split($2, d, /,/)
  for (i=1; i<=n; ++i) print d[i] }' parameters.txt |
while read -r number; do
   date -d "$number days ago"
done

This should do it:

#!/bin/bash

filename="parameters.txt"

while read -r line; do 
   # Look for the correct line (if you have other parameters)                                                                                                                                                                             
   if [[ $line == Days:* ]]; then 
      # Remove line prefix
      days=$(echo "$line" | cut -c 7-); 
      # Change the delimiter
      IFS=', '; 
      read -ra arr <<< "$days"
      for i in "${arr[@]}"; do
         # Check for current day
         if (( $(date +%d) ==  i )); 
            then echo "Found current day"; 
         fi; 
      done; 
   fi; 
done < $filename

Beware that changing IFS may cause you some problems if you have to make other read inside the script, in this case I advise you to change it back to its original value.

If your parameters.txt only contains the days parameter you can do

#!/bin/bash

filename="parameters.txt"
# Remove line prefix
days=$(cut "$filename" -c 7-); 
# Change the delimiter
IFS=', '; 
# etc ...

awk 'BEGIN { 
             split(strftime(),dat," ");
             day=dat[3]
            } 
     /^Days/ { 
              split($2,arr,",");
              for (i in arr) 
                          { 
                           if (day ==arr[i]) 
                                           { 
                                             print "True" 
                                           }  
                           } 
              }' parameters.txt    

One-liner

awk 'BEGIN {split(strftime(),dat," ");day=dat[3]} /^Days/ { split($2,arr,",");for (i in arr) { if (day ==arr[i]) { print "True" }  } }' parameters.txt

We begin by getting today's date using awk's strftime function and then further splitting this into a dat array using awk's split function and " " as the delimiter. We then set day to the 3rd index of the array. Then when we encounter a line beginning with Days, we split the section after the space ($2) into an array arr based on commas as the delimiter. We then loop through this array comparing the days with the current day and print True if there is a match