For the first video, you just need to know that in the 6th of January, the Orthodox Church blesses the waters, by dropping a wooden cross in the water (lake, river or sea). Then, a couple of young men (usually with far-right, pro-military tendancies) dive into the cold water and swim, till the first guy catches the cross.
The winner is blessed by the priest and is given some kind of a symbolic present (in the past, it wasn't so symbolic).

It's my favourite tradition, considering its complicated nature guarantees that we'll enjoy watching some funny accidents, like seagulls trying to consume the cross, priests falling into the water and crosses insisting on getting lost:


Here, I'm afraid that knowledge of the Greek language is almost necessary. It's a political advertizement for a female candidate of the Independent Greeks:


That's an advertizement for the party Greek Popular Democratic Liberation: