I am would like to know if you can see pleiades and sirius on the sky at the same time from Greece? What time of the year would that be? And I would be very greatful if someone knows some interactive starmap where I could also see how that would look like. Many thanks.
Stellarium is a free planetarium software, and it allows you view the position of stars in the sky at any date.
Sirius and the Pleiades are actually rather close in the sky. On the line of Orion's belt, on opposite sides. So for much of the the winter months, both are visible in the sky. For example in mid-December, they will be visible at midnight, in south-western skies. At the time of writing (8th October) they will be visible in the early morning.
It is a well known starscape, with Orion central, the Pleiades and the V of the Hyades on one side, and Sirus, scorching on the other.
Of course, the Pleiades and Sirius are "fixed stars", so the separation between them doesn't change. There is no time when they approach each other.
The simple answer is yes. Look around midnight in the South Easterly direction and you should be able to see them both at around this time of year. As James Kilfiger says above, Orion is your friend.