I received a document where it it specified that the flight relevant for such document can only take place in "Day VFR" conditions undear EASA regulations.
Is this connected to VMC? What does EASA defines as "Day VFR"?
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It's by opposition to Night VFR (NVFR) which allows to fly by night in visual meteorological conditions. So the answer will focus on NVFR, rather than on day VFR.
EASA harmonization over EU countries is done by enforcing Standardised European Rules of the Air (SERA). SERA states that a night flight must be conducted in IFR, and therefore the crew must hold an Instrument Rating.
Prior to SERA, some countries did not allow VFR at night for security reasons (aircraft not visually identifiable, and no transponder), but other did, that's the case for the UK that I'll use as an example here.
'Night' means the time from half an hour after sunset until half an hour before sunrise [...] at surface level.
In countries where NVFR is not allowed, IFR applies at night.
VFR @ night in Germany requires a HG and DG, VSI, - a standard IFR panel plus a VOR receiver, lights and License. The funny description for the beginning of night applies here too. The German Weather Service makes these times public for selected airports. Choose the one closest to yours. See this link (in German).
Else a flightplan is required and communication to radar. IMHO all overly too many requirements. VFR @ night is not so much more demanding as VFR/day on a hazy day. Considering it IFR because of the "lack of a horizon" shows that these rules are made by folks who never flew at night, or even never flew.