As a new feature on the blog, we are compiling a Language Calendar (see the bar under the banner). So far it contains only two dates:
- 4 March: National Grammar Day in the US
- 22 April: Modern English Usage Day (the day when Fowler’s usage guide was first published in 1926)
Please tell us if you know of any more days for the Calendar!
Four Five Nine (!) dates have already been added to these two: check out the Calendar for them. You will see there that Russian Language Day is the next language day on our calendar at the moment: 6 June.
It appears that, with two English Language Days, the English language is particularly popular in this respect. Any other languages, apart from (overwhelmingly) English, French, Russian, Arabic, Spanish and Korean? Is there a Dutch language day, for instance, or an Icelandic one? We’d love to hear.
Bridging the Unbridgeable 
I suggest Hangul Day http://en.wikipedia.org/wiki/Hangul_Day
Well, in 2010 the United Nations have established 23 April as English Language Day (as one of six official language days “to celebrate multilingualism and cultural diversity as well as to promote equal use of all six official languages throughout the Organization”, according to their website).
On 26 September, the European Day of Languages is celebrated. http://edl.ecml.at/
20 mars: journée internationale de la francophonie http://french.about.com/od/francophonie/a/dayoffrancophonie_2.htm
21 February – International Mother Language Day http://www.unesco.org/new/en/education/themes/strengthening-education-systems/languages-in-education/international-mother-language-day/
6 June – Russian Language Day (Alexander Pushkin’s birthday)
12 October – Spanish Language Day (Dia de la Hispanidad)
18 December – Arabic Language Day
20 April – Chinese Language Day