Skip to content

Celtic Tree Calendar · Month 5 of 13

Willow, the month of the letter Sail

Willow is the fifth of Graves' tree months, 15 April to 12 May, led by the Ogham letter Sail, S. Its Old Irish name sail genuinely means willow, and its medieval kenning, hue of the lifeless, points to the tree's pale colour.

Is this your tree month?

Enter your birth date to see your free Celtic Tree chart. No email needed.

Not saved, and never put in a web address. Privacy policy

The Ogham letter Sail drawn upright on a stemline, four strokes to the right of the stemline, read from the bottom up; beside it the words Sail, the Willow month, letter S, and the dates 15 April to 12 May. Sail The Willow month · letter S 15 Apr to 12 May Old Irish sail Name means willow Read from the bottom up
Sail, the letter S, as it is cut on the stones: four strokes to the right of the stemline, read upwards. Drawn by us from its Unicode code point, not from a font.

Facts

Tree month
15 April to 12 May (Graves, 1948)
Days
28
Ogham letter
Sail, the letter S (U+1684, OGHAM LETTER SAIL)
Old Irish name
sail; Graves spells it Saille
The name means
willow
Letter and tree
The name means the tree
Kenning (Morann)
lí n-aimbi, "hue of the lifeless" (tr. Calder 1917, p. 279)
Medieval tree gloss
willow (Auraicept, Calder 1917, p. 91)
The tree in Modern Irish
saileach

Meaning

Willow grows by water, bends in wind and flood, and roots again from a cut branch. The kenning of its letter is "hue of the lifeless", in Calder's translation, and the Auraicept explains it by the pale colour of the tree: "The colour of a lifeless one … owing to the resemblance of its hue to a dead person".

Our reading takes receptiveness and recovery from the tree: people born in this month are read as sensitive and adaptable, with deep feelings and the ability to recover after hard seasons, and yielding is read as a kind of strength. The reading is ours.

The letter

Sail is the fourth letter of the stones: four strokes to the right of the stemline. Its Old Irish name sail genuinely means willow, and both medieval tracts gloss it so, as O'Flaherty did in 1685. Graves spells it Saille, a spelling many websites keep; the Irish name is Sail.

The willow and the Moon

Many pages call the willow the Moon's tree in Celtic tradition. The link is real, but it is not Irish. Nicholas Culpeper's herbal of 1652 puts the willow under the Moon as English plant lore, Graves pairs the willow with Monday among his trees of the week, and Paterson gives the Willow month the Moon as its planet. No medieval Irish source does. We give no planet to any month.

In Graves' year

Willow runs from 15 April to 12 May, the same in every list of the tree months. The gemstone, animal and personality title that tree-sign websites give it are modern, after Paterson (1992) and later lists, and are not used here.

Correspondences

Each item with how it stands: “sourced” is in the work cited, “our construction” is EsoTerrier’s own, and “disputed” means the sources disagree.

The Auraicept's explanation
"The colour of a lifeless one ... owing to the resemblance of its hue to a dead person" (Calder 1917, p. 91) sourced
The Ogam Tract's gloss
the same tree (Calder 1917, p. 277) sourced
O'Flaherty's gloss (1685)
Sail, the willow sourced
How the letter is tied to the tree
The name means the tree sourced
Colour and bird words for the letter (a medieval curiosity)
Colour: sodath, "fine-coloured"; bird: seg, "hawk". Words that begin with the letter, like the tract's lists of churches; not the tree's colour or animal sourced
Our reading
A tree that bends in flood and roots again from a cut branch: sensitive and adaptable, with yielding read as strength. our construction

How the schools read it

The medieval tracts
The kenning "hue of the lifeless"; the Auraicept glosses the letter willow.
O'Flaherty, 1685
Sail, the willow.
Graves, 1948
The fifth month, 15 April to 12 May.
Ogham scholarship
sail is one of the few letter names that genuinely names a tree.
Tree-sign websites
The same dates; a planet, gemstone, animal and personality title, all modern. Not used here.
EsoTerrier
A tree that bends in flood and roots again from a cut branch: sensitive and adaptable, with yielding read as strength.

Claims about it

  • The willow is the Moon's tree in Celtic tradition.

    unverified

    What the record shows: A modern Pagan motif. Culpeper's herbal of 1652 puts the willow under the Moon as English plant lore, and Graves pairs it with Monday; no Irish source does.

  • The Willow month belongs to an old Celtic calendar that the Druids kept.

    false

    What the record shows: No ancient or medieval source has a tree calendar or a tree for each birthday. The thirteen months are Robert Graves', set out in 1948.

  • The Willow month has a traditional personality title.

    false

    What the record shows: The thirteen titles come from one website of the 2000s and were copied widely; nothing in Graves, Paterson or the medieval tracts supports them. We never use them.

  • The Willow month has a traditional planet, element, gemstone and animal.

    false

    What the record shows: Modern additions: the planets are Helena Paterson's (1992), the rest come from later web lists that disagree with one another. None is medieval or in Graves as a set.

Elsewhere on EsoTerrier

Sources

Primary texts (5)
  1. George Calder (ed. and tr.), Auraicept na n-Éces: The Scholars' Primer, with In Lebor Ogaim (the Ogam Tract). Edinburgh: John Grant, 1917. Quoted with page. archive.org/details/auraicept00calduoft
  2. Roderick O'Flaherty, Ogygia. London, 1685; English by James Hely, Dublin, 1793. archive.org/details/ogygiaorachrono00helygoog
  3. Robert Graves, The White Goddess. London: Faber and Faber, 1948 (revised 1952 and 1961). Facts only; not quoted.
  4. The Unicode Standard, Ogham block U+1680 to U+169F (encoded in Unicode 3.0, 1999). unicode.org/charts/PDF/U1680.pdf
  5. Nicholas Culpeper, The English Physitian (1652), as The Complete Herbal. gutenberg.org/ebooks/49513
Studies and other sources (9)
  1. Damian McManus, "Irish letter-names and their kennings", Ériu 39 (1988), pp. 127 to 168. jstor.org/stable/30024135
  2. Deborah Hayden and David Stifter, "Ogam and Trees", OG(H)AM project, 20 December 2022. ogham.glasgow.ac.uk/index.php/2022/12/20/ogam-and-trees
  3. "Ogham", Wikipedia (orientation only). en.wikipedia.org/wiki/Ogham
  4. Mary Jones, "Celtic Tree Calendar", Jones's Celtic Encyclopedia (2004). maryjones.us/jce/celtictreecalendar.html
  5. Christopher Penczak, "Celtic Astrology Tree Signs" (2013), crediting Paterson. christopherpenczak.com/celtic-astrology-tree-signs
  6. Helena Paterson, The Celtic Lunar Zodiac. London: Rider, 1992 (known from catalogue records and Penczak). search.worldcat.org/oclc/31606966
  7. whats-your-sign.com, "Celtic Tree Astrology", Wayback Machine capture of 28 February 2008 (cited only as the source of the claim). web.archive.org/web/20080228140902/http://www.whats-your-sign.com/celtic-tree-astrology.html
  8. Icy Sedgwick, "Celtic Tree Astrology" (2018), a composite gemstone and animal list (cited only as the source of the claim).
  9. Foras na Gaeilge, focloir.ie and teanglann.ie (Modern Irish forms). focloir.ie
Our own research (2)
  1. EsoTerrier, Celtic Tree research and validation, 11 to 13 October 2026.
  2. EsoTerrier calculation engine, the Celtic Tree Calendar service, its texts and its documentation.

Your Celtic Tree reading, $3.99. Includes this system's raw data. Written for you by AI. Read a sample. Charts and the written text are free in every account.

By EsoTerrier. Written with AI assistance from our research and checked against its sources; last revised 19 October 2026. How we write these pages.

First published ; last revised . Corrections: support@esoterrier.com.