June 28 marks the anniversary of the signing of Ukraine's post-Soviet constitution. Tymoshenko, imprisoned since 2011, said she had "mixed feelings" about the day.
"For the first time I don't want to congratulate anyone with this holiday," she said in a statement. "On the contrary, I want to express my deepest sympathy to all of us, and mostly our Constitution. "
Tymoshenko was sentenced to seven years in prison in 2011 for abusing her position when she helped broker a natural gas deal with Russian company Gazprom. The government said the deal struck a major blow to a weakened Ukrainian economy. Leader of a revolution in 2005, her supporters suspect the charges against her are politically motivated.
The European Union expressed frustration with Ukraine's justice system this week. Ukraine longs for a closer relationship with the EU, though the 27-member bloc says Kiev can improve on its political and democratic principles.
EU representatives this year said the bloc was ready to sign an association agreement with Ukraine provided the government addresses a series of political concerns.
Tymoshenko's trial in 2011 began the day after Constitution Day.