grammar.nouns.plural-invariable
Nouns that don't change in the plural
Unstressed vowel + s stays put: el lunes / los lunes, la crisis / las crisis, el paraguas / los paraguas. Only the article marks number.
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Explanation
Some nouns have the same form in the singular and the plural, only the article (or context) marks number. The main group is words ending in an unstressed vowel + s: el lunes / los lunes, la crisis / las crisis, el análisis / los análisis, el paraguas / los paraguas.
This covers the weekdays (martes, miércoles…), many Greek-derived -is words (la tesis, la dosis, el énfasis), and verb-noun compounds (el cumpleaños, el sacacorchos, el abrelatas).
So the article does the counting: el lunes means 'on Monday', los lunes means 'on Mondays'; una crisis vs varias crisis. (Words ending in a stressed vowel + s do change: el país → los países.)
Examples
On Mondays I work from home.
Region: global
There were several crises that year.
Region: global
On Wednesdays there's a fair and on Saturdays, a market.