What causes "Could not find day of week for ' '" in languagetool-org/languagetool?

Add the missing inflected/abbreviated form to the appropriate branch of getDayOfWeek

Could not find day of week for '<dayStr>'

Error message

Could not find day of week for '<dayStr>'

What it means

The Polish DateCheckFilter maps Polish weekday tokens (pn/pon, wt, śr, czw, pt/piątk…, sob, niedz…) to Calendar constants. getDayOfWeek throws this RuntimeException when the token matches none of these patterns. The Polish mapping requires correctly accented input (śr, piątk), so encoding or deaccenting issues can cause misses.

Source

Thrown at languagetool-language-modules/pl/src/main/java/org/languagetool/rules/pl/DateCheckFilter.java:46

public class DateCheckFilter extends AbstractDateCheckFilter {

  @Override
  protected Calendar getCalendar() {
    return Calendar.getInstance(Locale.forLanguageTag("pl"));
  }

  @SuppressWarnings("ControlFlowStatementWithoutBraces")
  @Override
  protected int getDayOfWeek(String dayStr) {
    String day = dayStr.toLowerCase();
    if (day.startsWith("pon")) return Calendar.MONDAY;
    if (day.startsWith("wt")) return Calendar.TUESDAY;
    if (day.startsWith("śr")) return Calendar.WEDNESDAY;
    if (day.startsWith("czw")) return Calendar.THURSDAY;
    if (day.equals("pt") || day.startsWith("piątk") || day.equals("piątek")) return Calendar.FRIDAY;
    if (day.startsWith("sob")) return Calendar.SATURDAY;
    if (day.startsWith("niedz")) return Calendar.SUNDAY;
    throw new RuntimeException("Could not find day of week for '" + dayStr + "'");
  }

  @Override
  protected String getDayOfWeek(Calendar date) {
    return date.getDisplayName(Calendar.DAY_OF_WEEK, Calendar.LONG, Locale.forLanguageTag("pl"));
  }

  @SuppressWarnings({"ControlFlowStatementWithoutBraces", "MagicNumber"})
  @Override
  protected int getMonth(String monthStr) {
    String mon = monthStr.toLowerCase();
    if (mon.equals("stycznia") || monthStr.equals("I")) return 1;
    if (mon.equals("lutego") || monthStr.equals("II")) return 2;
    if (mon.equals("marca") || monthStr.equals("III")) return 3;
    if (mon.equals("kwietnia") || monthStr.equals("IV")) return 4;
    if (mon.equals("maja") || monthStr.equals("V")) return 5;
    if (mon.equals("czerwca") || monthStr.equals("VI")) return 6;
    if (mon.equals("lipca") || monthStr.equals("VII")) return 7;

View on GitHub (pinned to 2e990059ce)

Solutions

Add the missing inflected/abbreviated form to the appropriate branch of getDayOfWeek
Verify input encoding/normalization so 'ś' and 'ą' survive to the filter (UTF-8)
Align the rule's regex with the exact token forms the mapping recognizes

Example fix

// before
if (day.startsWith("niedz")) return Calendar.SUNDAY;
throw new RuntimeException("Could not find day of week for '" + dayStr + "'");
// after
if (day.startsWith("niedz")) return Calendar.SUNDAY;
if (day.equals("pon") || day.startsWith("poniedzial") || day.startsWith("poniedział")) return Calendar.MONDAY; // add missing forms as needed
throw new RuntimeException("Could not find day of week for '" + dayStr + "'");

Defensive patterns

Strategy: validation

Validate before calling

if (dayStr == null || !dayStr.matches("(?i)(pn|pon.*|wt.*|śr.*|czw.*|pt|piątk.*|piątek|sob.*|niedz.*)")) throw new IllegalArgumentException("Unrecognized Polish weekday: " + dayStr);

Type guard

boolean isPolishWeekday(String s) { String d = s == null ? "" : s.toLowerCase(); return d.startsWith("pn")||d.startsWith("wt")||d.startsWith("śr")||d.startsWith("czw")||d.equals("pt")||d.startsWith("piątk")||d.startsWith("sob")||d.startsWith("niedz"); }

Try / catch

try { day = filter.getDayOfWeek(dayStr); } catch (RuntimeException e) { log.warn("Polish weekday unmatched: {}", dayStr); /* ignore match */ }

Prevention

Ensure UTF-8 handling end-to-end so 'ś'/'ą' are not mangled to 's'/'a'
Test inflected weekday forms your regex can produce
Add every regex alternative to a mapping-coverage test

When it happens

Trigger: A rule matches a weekday token not covered by the patterns (e.g. 'pon' is covered via pn/pon branch — but variants like 'poniedziałek' partial inflections or 'sobota' inflected forms 'soboty' are handled by startsWith; anything else such as an unaccented 'sr' instead of 'śr' throws).

Common situations: Text with ASCII-folded Polish characters ('sr', 'piatek') from other processing steps; rule regex extended with new inflected forms not handled in the mapping.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of languagetool-org/languagetool@2e990059ce (2026-09-06). Data as JSON: /api/errors/68f4185339578442. Report an issue: GitHub.