Programmatically add a translation of month I need to add a custom translation of long form of months. The existing translation must be changed slightly.

One way to do it is via UI, however, not all months are available for translation yet (this is a new site). I would like to write a function in a module that would add all translations of months programmatically (and it would also be possible to use it on other sites). Is there a way to do it?
Use string overrides directly in either your settings.php file or through the string overrides module. See this note for specifics, this also lets you migrate this string overrides snippet from site to site either through simple cut and paste or through the string overrides migrate extension. This also deals with strings not yet known to the locale UI through t().

Example: 

 $conf['locale_custom_strings_en'][''] = array(
   'January'      =&gt; 'New string 1',
   'February'      =&gt; 'New string 2',
 );