A normal month can look comfortable because the car service, school costs, insurance, gifts and repairs have not arrived yet.
Look back across a year and add the costs that appear quarterly, twice a year or annually. For truly irregular costs, estimate a reasonable yearly total.
A useful question
What expense would make this scenario feel wrong if it arrived next week?
Enter recurring costs at their real frequency. Use Yearly for an estimated annual total when that is the clearest honest picture.
For more on building a budget around real life, see The Art of the Budget.