1. when we have reservation in that period (in other words, we don't have free capacity)
2. when capacity is less then one
3. when there is no price in calendar (prices isn't entered in calendar)
4. when price is 0
5. when CHECK IN is forbidden
6. when day is blocked for selling (enable=N)
7. when minimum advance reservation (days or hours in advance when reservation can be made) had expired.
8. when option CHANGE OVER DAY is turned on (shift).
Exception: when there is dafault price (it is only visable in calendar) but it doesn't affect on price.