Have a memory/mind like a sieve
Definition 1 of 1
Pro Tip 1/3Casual Usage
Used in informal contexts among friends or in relaxed settings, but not in formal situations. 
Sorry I missed your party, I have a mind like a sieve and forgot the date!
Pro Tip 2/3Self-Deprecating
Often used humorously to admit one's own forgetfulness without being too harsh on oneself. 
I'd forget my head if it wasn't attached, I've got a memory like a sieve!
Pro Tip 3/3Not Literal
It's an exaggeration and not to be taken literally; it doesn't mean someone forgets everything. 
I know I have a mind like a sieve, but I do remember your birthday!