Same with the shva or pasach under a lamed or beis.
Rule of the thumb: pasach is short for לה, so if the thing referred to is a known thing (ה הידיע) it's a pasach (e.g. ביום ההוא, למלך, למדינות). If not (or if it's expressly described) , it's a shva (e.g. ביום אחד, ביום שלשה עשר).