When flying from Bangkok to Saint Petersburg, the time of day you decide to fly out makes little to no difference towards the overall price you’ll pay on tickets.
For a flight from Bangkok to Saint Petersburg, you will be flying into Pulkovo. Saint Petersburg Pulkovo (LED) is just 14.0 km from the centre vof the city. Pulkovo is the only Saint Petersburg airport and it handles 0 inbound flights from Bangkok per day. 0 airlines have flights to Saint Petersburg Pulkovo from Bangkok on a regular basis.
These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Bangkok Suvarnabhumi Airport to Saint Petersburg Pulkovo Airport flight would often be abbreviated as a BKK to LED flight. Bangkok Don Mueang Intl Airport (DMK), Rayong Utapao Rayong Pattaya Airport (UTP), Hua Hin Airport (HHQ) are other airports near this flight route and their unique identifiers/IATA codes.