According to the usage of the Dúnedain, the fourth month of the year, equivalent to QuenyaViressë. The meaning is obscure: the name arguably contains a root-word for 'web', though it is unclear how that idea relates to the nature of the month. The Shire-hobbits called the same month 'Astron'.
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The derivation of SindarinGwirith is not well understood, though it apparently shares linguistic roots with the equivalent Quenya month name Víressë. The origin of Víressë is not in itself fully explained, but it appears to be connected with words for youth or new growth, which makes sense given that the month occurred during the season of spring.