Last month’s rule? Apostrophes are NEVER ever used to denote plurals.
This month’s? Apostrophes are used to denote possession.
For example:
If there are two or more dogs, companies or Johns, the apostrophe comes after the ‘s’.
But, there is no apostrophe in ours, yours or hers.
Neither is there an apostrophe in ‘its’ when it is (it’s) showing possession.
Hopefully, this month we’ve managed to avoid our own ‘deliberate’ mistakes.