Set family savings goals for holidays and emergencies.
Working out a savings contribution is not simply the goal divided by the months, because two things earn interest along the way: the balance you already hold, and each contribution from the moment it lands. The existing balance is grown forward at the monthly rate, and the remaining gap is solved as an ordinary annuity — the standard future-value factor of ((1+r)^n - 1) / r. Over eighteen months at a modest rate the interest is small, but the structure matters as the horizon lengthens: on a ten-year goal, interest can cover a fifth of the target. Interest also rewards starting rather than waiting, which is what the projected value of current savings makes visible.
Family Savings Goal
Existing grown = saved x (1 + r)^n; monthly = (goal - existing grown) / (((1 + r)^n - 1) / r), where r is the monthly rate and n the months
Because both your existing balance and each monthly payment earn interest. At low rates over short periods the difference is small, but it grows quickly with time.
Somewhere capital-secure and accessible — an instant-access or notice savings account. Anything under about three years should not be exposed to market risk, since a fall at the wrong time cannot be recovered before the date.