No idea what an sla day is an no idea if that field contains any values or if you're updating it with this solution. However, any time that holidays are involved you need a table to identify all the holidays along with their dates and populate it with records. When you run out of records, things usually stop working properly. This is because holiday dates are not universal, nor are they all consistent.
Maybe one or both of these functions will do the trick for you
http://access.mvps.org/access/datetime/date0006.htm
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.