Introduction

In which yes- and -day in yesterday are the same root.

• PIE *dʰǵʰes-tero-dʰoǵʰes
• Germ. *(d)ges-tera-dagaz
• OE ġes-ter-dæġ
• English yes-ter-day

"The day that’s the other day"

Germanic *dagaz is often explained as being from *dʰegʷʰ-: "to burn", but the loss of labialization would be irregular, and the semantic connection is questionable. But if you assume that *dʰǵʰes-: "yesterday" (widely attested) is the zero-grade of a mostly unattested *dʰeǵʰ-es-, then there would be an expected o-grade *dʰogʰ-es, which is a perfect fit for Germanic *dagaz. (I'm working from the van den Oever/Kloekhorst/Klimp theory of *dagaz.)

Teaser

day, daisy, yesterday, Camelops hesternus

Full Text

• Proto-Indo-European *dʰeǵʰ- day?, repeat?

  • Proto-Indo-European *dʰeǵʰnos daily?, repeating?

    • Balto-Slavic

      • East Baltic

        • Lithuanian dãžnas frequent
      • West Baltic

        • Old Prussian *desnai

          • Old Prussian kudesnammi periodically, every so often
  • Proto-Indo-European *dʰeǵʰ-es

    • Proto-Indo-European *dʰoǵʰ-es

      • Germanic *dagaz day, the rune ᛞ

        • East Germanic

          • Gothic 𐌳𐌰𐌲𐍃 dags day, the letter 𐌳

            • Crimean Gothic tag day
            • Gothic *Dagisþius Day-People? (personal name)

              • Greek Δαγισθαῖος Dagisthaîos
        • North Germanic

          • Old Norse dagr

            • Danish dag
            • Icelandic dagur
        • West Germanic

          • Old English dæġ day, the rune ᛞ

            • English day
            • Old English dæġes ēage daisy lit. "day's eye"

              • English daisy
          • Frankish

            • Dutch dag
          • Old High German tag

            • German Tag
            • Yiddish טאָג tog day
        • English dagaz the rune ᛞ
        • Germanic *gesteradagaz yesterday see below
    • Proto-Indo-European *dʰǵʰes- petrified genitive singular?

      • Proto-Indo-European *dʰeǵʰesr̥ r/n stem noun

        • Indo-Iranian *áȷ́ʰr̥ loss of *dʰ- by influence from a zero-grade form?

          • Indo-Aryan

            • Sanskrit 𑀅𑀳𑀭𑁆 áhar day

              • Sanskrit 𑀲𑀧𑁆𑀢𑀸𑀳 saptāhá seven-day, week

                • Pali 𑀲𑀢𑁆𑀢𑀸𑀳 sattāha seven-day, week

                  • Khmer សត្តាហៈ sattaahaʼ week
                • Sauraseni

                  • Pahari

                    • Nepali साता sātā week
                • Hindi सप्ताह saptāh week
                • Tamil ஸப்தாஹம் saptāham week
                • Thai สัปดาห์ sàp-daa week
          • Iranian

            • Avestan 𐬀𐬌𐬌𐬀𐬭 aiiar day
        • Proto-Indo-European *dʰǵʰéni locative

          • Indo-Iranian *áȷ́ʰani

            • Indo-Aryan

              • Sanskrit 𑀅𑀳𑀦𑀺 áhani
      • Proto-Indo-European *dʰǵʰési locative

        • Albanian dje yesterday
        • Celtic *gdesi

          • Brythonic

            • Welsh ddoe yesterday
          • Celtic *sindos gdesi this yesterday

            • Old Irish indé

              • Irish inné yesterday
        • Hellenic *kʰtʰés

          • Ancient Greek χθές khthés yesterday

            • Greek χθες chthes yesterday, the past
        • Italic *hezī

          • Latin heri yesterday

            • Eastern Romance

              • Romanian ieri yesterday
            • Western Romance

              • French hier yesterday

                • Haitian Creole yè yesterday
              • Italian ieri
              • Spanish ayer yesterday
            • Latin abante heri day before yesterday

              • Western Romance

                • French avant-hier day before yesterday
                • Italian avantieri day before yesterday
                • Spanish anteayer day before yesterday
      • Proto-Indo-European *dʰǵʰesteros with contrastive *-teros

        • Germanic *gesteraz yesterday

          • West Germanic

            • Old English ġiestran

              • English yester-
            • Frankish

              • Dutch gisteren yesterday
            • Old High German gesteren

              • German gestern yesterday
          • Germanic *gesteradagaz yesterday

            • East Germanic

              • Gothic 𐌲𐌹𐍃𐍄𐍂𐌰𐌳𐌰𐌲𐌹𐍃 gistradagis tomorrow
            • West Germanic

              • Old English ġiestrandæġ yesterday

                • English yesterday
            • North Germanic

              • Old Norse

                • Swedish gårdag yesterday
        • Italic *hesternos

          • Latin hesternus related to yesterday, yesterday's

            • Translingual Camelops hesternus Yesterday's Camel (species name)
    • Proto-Indo-European *dʰǵʰyes-

      • Germanic *gēz yesterday

        • North Germanic

          • Old Norse gær yesterday

            • Old Norse í gær yesterday "at yesterday" compare locative *dʰǵʰés-i

              • Danish i går yesterday
              • Icelandic í gær yesterday, last night
        • West Germanic

          • Old English ġeāra long ago ^[1]

            • English yore ^[1]
      • Indo-Iranian *ȷ́ʰyás

        • Indo-Aryan *źʰyás

          • Sanskrit 𑀳𑁆𑀬𑀲𑁆 hyás yesterday

            • Elu

              • Sinhala ඊයේ īyē yesterday
            • Pali hiyyō
            • Sauraseni 𑀳𑀺𑀚𑁆𑀚𑁄 hijjo

              • Nepali हिजो
            • West Indo-Aryan

              • Romani idž yesterday
        • Iranian *ȷ́yáh

          • Avestan 𐬰𐬫𐬋 zyō
          • Western Iranian

            • Northwestern Iranian

              • Kurdish duhî yesterday
            • Old Persian 𐎮𐎹𐎣 di-ya-ka

              • Persian دی di yester, yesterday

                • Persian دیروز diruz yesterday
                • Persian دیشب dišab last night
      • Hellenic

        • Ancient Greek χθιζός khthizós related to yesterday, yesterday's

Visual

Collected English words

day, daisy, dagaz, yester-, yesterday, Camelops hesternus, yore

Footnotes

1. ^

   Old English merges Germanic *je- and *ge- both to ġe-. Most likely the Old English "ġeāra" represents Germanic *je-, and "yore" is connected "year". But if it represents Germanic *ge-, then it strongly resembles Old Norse "gær": "yesterday". Either options is semantically plausible.