# Calculate days on which Friday the 13th occurs (inspired from the work of the user justsomeguy)

for i in {2018..2025}-{01..12}-13; do [[ \$(date --date \$i +"%u" | grep 5) != 5 ]] || echo "\$i Friday the 13th"; done
Friday is the 5th day of the week, monday is the 1st. Output may be affected by locale.
Sample Output
```2018-04-13 Friday the 13th
2018-07-13 Friday the 13th
2019-09-13 Friday the 13th
2019-12-13 Friday the 13th
2020-03-13 Friday the 13th
2020-11-13 Friday the 13th
2021-08-13 Friday the 13th
2022-05-13 Friday the 13th
2023-01-13 Friday the 13th
2023-10-13 Friday the 13th
2024-09-13 Friday the 13th
2024-12-13 Friday the 13th
2025-06-13 Friday the 13th```

2
2018-07-10 21:31:02

## 4 Alternatives + Submit Alt

• I removed the dependency of the English language Show Sample Output

5
for y in \$(seq 1996 2018); do echo -n "\$y -> "; for m in \$(seq 1 12); do NDATE=\$(date --date "\$y-\$m-13" +%w); if [ \$NDATE -eq 5 ]; then PRINTME=\$(date --date "\$y-\$m-13" +%B);echo -n "\$PRINTME "; fi; done; echo; done
· 2018-06-25 09:20:57
• Simply change the years listed in the first seq, and it will print out all the months in that span of years that have Friday the 13ths in them. Show Sample Output

1
for y in \$(seq 1996 2018); do echo -n "\$y -> "; for m in \$(seq 1 12); do NDATE=\$(date --date "\$y-\$m-13" +%A); if [ \$NDATE == 'Friday' ]; then PRINTME=\$(date --date "\$y-\$m-13" +%B);echo -n "\$PRINTME "; fi; done; echo; done
· 2018-06-13 20:11:46
• Alter the years in the first brace expansion to select your year range. Modify date format to your liking but leave " %w" at the end. Show Sample Output

0
for i in {2018..2022}-{01..12}-13; do date --date \$i +"%Y %B %w" | sed '/[^5]\$/d; s/ 5*\$//'; done
· 2018-07-09 15:47:39
• Removed grep and simplified if statement. -- Friday is the 5th day of the week, monday is the 1st. Output may be affected by locale. Show Sample Output

0
for i in {2018..2025}-{01..12}-13; do [[ \$(date --date \$i "+%u") == 5 ]] && echo "\$i Friday the 13th"; done
· 2018-07-30 15:54:24

### What do you think?

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