Three-part modeling: end-of-month bank balance

Modeling subject: the bank balance on the last day of each month. The salary is the same every month, yet some months there's a surplus and some months it goes negative — what determines the balance?

Build 'end-of-month bank balance' as a three-part model: input → relationship → output. (1) Output: define in one line what you're trying to predict (e.g., the balance of the main account as of the last day). (2) Input: pick only the 2–4 variables that move the output the most. (3) Relationship: write in one sentence how the inputs combine into the output. You may write it in words or as a rough formula. Finally, you must state at least one 'variable you deliberately excluded' and why, plus one 'situation where this model won't fit well.'