diarrhea regurlarly,all night.

Keep a record of your diet, and of the occurences of these nights. Then take it to your doctor.

Possible causes:
Too much fat in foods
Too much sugar
Big changes in diet
Some foods are laxatives (salads if you're not used to it)
Lactose Sensitivity/Intolerance
Gall bladder
Irritable Bowel Syndrome
Celiac disease/gluten sensitivity
Certain genetic tendencies
Some medications
Certain allergies/sensitivities

Also, if the OP is female, your hormones contribute to digestion too so see if this coincides with your hormone fluxuations (i.e. ovulation and period.) I used to get BAD stomach cramps & diarrhea regularly too, two nights before my period.

The IBS mentioned above is pretty common, though normally its more frequent and throughout the day.

Honestly I feel like either you A) have a habit of eating something you're sensitive to, only at dinner, and only twice a month, or B) its something to do with your hormones (again, assuming you're female.) Your hormones may be swinging way too wildly.