A personal use day is any day that you're using the property, or a day that anyone else is using the property for less than a fair rental price or a day that your relative is paying fair rent but not residing there. Personal use does not include a day spent "substantially full-time repairing and maintaining the property".
Any personal use days reduce your allowable rental expenses; all rental expenses must be divided between rental and personal use. So if you have 14 days of personal use of a vacation home and 150 days that the property was actually rented, and your expenses the property are $10,000, then you can deduct only $9,146 (14 + 150 = 164 -> 150 / 164 = 91.46%). If rental expenses exceed rental income then this rental loss can reduce your non-rental income like wages, interest, dividends, and capital gains.
However, if your personal use days are generally more than 10% of your days actually rented then the IRS will consider your rental property as a residence. Specifically, if your personal use days are more than the greater of 14 days or 10% of the days that you rent to others for a fair market price, then the property will be a residence. Therefore, you can have a maximum of 33 personal days, in year with 365 days, only if the property is rented for the remaining 332 days, before your property is considered a residence.
Once your property is considered a residence, then if rental expenses exceed rental income then the loss cannot reduce your non-rental income.
Now, if the property is considered a residence but your days actually rented are less than 15 days, then you aren't to report the rental income or rental expenses. You would, however, deduct the property taxes and mortgage interest on Schedule A just like your actual residence.