Update release-reminder workflow

Author: Sophie Wigmore

.github/workflows/release-reminder.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   determine-date:
-    name: Release buildpacks on 2nd and 4th Thursday of the month
+    name: Release buildpacks on 2nd and last Thursdays of the month
     runs-on: ubuntu-22.04
     outputs:
       should_run: ${{ steps.should_run.outputs.bool }}
@@ -20,14 +20,16 @@ jobs:
             echo "bool=true" >> "${GITHUB_OUTPUT}"
           else
             day_of_month=$(date +%d)
-            # Check if it's the second or fourth Thursday of the month
+            last_day_cutoff=$(expr $(date -d "-$(date +%d) days month" +%d) - 6)
+            # Check if it's the second or last Thursday of the month
             # second thursday of the month will always be between day 8 and 14 (inclusive)
             if [[ "$day_of_month" -ge "8" && "$day_of_month" -le "14" ]]; then
                 echo "It's the second Thursday of the month"
                 echo "bool=true" >> "${GITHUB_OUTPUT}"
-            # fourth thursday of the month will always be between day 21 and 28 (inclusive)
-            elif [[ "$day_of_month" -ge "22" && "$day_of_month" -le "28" ]]; then
-                echo "It's the fourth Thursday of the month"
+            # last thursday of the month will always be within 6 days of the last day of the month
+            # $last_day_cutoff=(# days in this month - 6)
+            elif [[ "$day_of_month" -ge "$last_day_cutoff" ]]; then
+                echo "It's the last Thursday of the month"
                 echo "bool=true" >> "${GITHUB_OUTPUT}"
             else
                 echo "It's another Thursday of the month"