Its not ideal but when I wanted to do a 'spring sale' I used pro tools in calendar where I select the dates I want to discount and then set a rule for them. I choose a different discount for different lengths of stay (I.e. 3 nights 10% off, 5 nights 15% off, 7 nights 20% off). I then set a minimum stay for this discount set. I can then apply that rule set to all the dates I want to include.