People often ask how can i know if such and such a day is worth visiting an airport to do some plane spotting eg lets say you intend to visit Cambridge Airport on 1 October 2019 , First read this article that explains what a Notam is
https://en.wikipedia.org/wiki/NOTAM thats very helpful .
By looking up the Notams for the airport your going to visit sometimes you can save yourself a trip as the airport could be shut for a holiday or repairs etc so always lookup the notams the day before you intend to travel. Many people annoy the crap out of everyone in Facebook groups asking the same old question " what time should i be at the base and what will i see " , half the fun of spotting is to just show up and see things unexpectedly, even if i gave out times a specific plane was due nothing is set in concrete things can be early or late or cancel.
As the saying goes the early bird catches the worm so if planning to spend a day somewhere be there 8am to 6pm and chances are you will see 95% of everything flying that day. When it comes to Military bases/flights there's a thing called OPSEC so read this :
https://en.wikipedia.org/wiki/Operations_security its best not to give out exact times for Military flights, sometimes info does leak the day before a major movement eg a flight of fighter jets coming from the USA to the UK might get reported in forums or groups, in general you might know a day something may happen but not the exact time. Do not ask for exact times you will annoy the people running aviation forums or groups & by the way these are monitored by the Military and police.
here i will list a few of the websites i use to find Notam info :
NOTAM Notice to Airmen Guide Help explination for Plane Spotters Avgeeks https://www.youtube.com/watch?v=CmQt6NR5VKM so use that as much as you need to learn to read Notmas.
Dave