When an ‘s’ is added to the end of the word just to indicate that it is plural, there is no need to use an apostrophe. So in the case of ‘signs’, no apostrophe is necessary, as there is no ownership and no abbreviation.

One of the most common mistakes that people make when it comes to apostrophes is using them in plurals ending in an ‘s’. For example, we are open Monday’s to Friday’s. In order to pluralise a regular noun all you usually need is to add an ‘s’. Therefore, the correct version is we are open Mondays to Fridays.