Legal provisions for optional referendums at national level

Greece
Answer
Yes
Source
CONSTITUTION OFGREECE
(Adopted1975; last amended 2008)
Article 44
2.The President of the Republic shall by decree proclaim a referendum on crucial national mattersfollowing a resolution voted by an absolute majority of the total number ofMembers of Parliament, taken upon proposal of the Cabinet.
Comment
A referendum can becalled for crucial national issues and for existing laws which regulate importantsocial issues.