The tax year in Bulgaria is the calendar year

Residents of Bulgaria are subject to taxes in Bulgaria on their worldwide income and gains. Non Bulgarian residents are taxed on income and gains derived from Bulgarian sources only.

You are treated as resident in Bulgaria if either you are permanently domicile in Bulgaria; or you spend more than 183 days in Bulgaria within any 365 day period ending in the tax year in question.

 Foreign expats are taxed on their Bulgarian source income alone regardless of the amount of the number of days they spend in Bulgaria