The journalist Luke Harding, who has just left Donetsk, comments, "There is every possibility Russia will invade. The crunch moment will come after the separatist 'referendum' on May 11, and before May 25."
That long weekend from Victory Day on Friday 9th to the Donetsk "referendum" on Sunday 11th could be when it all reaches boiling point. Putin is planning to visit Crimea on the 9th too.
Ukraine has to retake control of the "People's Republic of Donetsk" before the 11th to stop the "referendum" going ahead. Equally, Russia has until the 25th to ensure that no undisputed vote can take place in eastern Ukraine. It needs to provoke such massive disruption that the legitimacy of the result will be called into question.