like night and day
So far this season, there have been 672 day games played in the major leagues, and with about 150 remaining on the schedules of the 30 major league teams, it looks like we're on pace for 822 day games this season, which would be the most day games played in any of the past 20 seasons. If you had known this at the beginning of the season you might have said "That's great - I know that Biggio gets hit by a pitch about once every 8.4 day games compared to about once every 10.7 night games - he'll break the record for sure!".
But it hasn't quite worked out that way - due to his reduced schedule Biggio has played in just 31 day games this season and started only 15 of them. With only 2 day games left on the schedule, this might be the first time season since 1991 in which Biggio was not plunked in a day game.
Even if he was playing every day game this season though, the league-wide rate of plunks per game is down 4% from last season, and down 9.3% in day games. Only 470 batters have been hit by pitches in day games this season, and an optimistic projection would bring the total to 575 by the end of the season. (I'll leave it up to you to determine whose idea of "optimism" that is). This looks like it will be the first year since 2002 in which plunks per night game will exceed day plunks per day game. There have only been 4 such seasons during the span of Biggio's career.
Overall, from 1988 through yesterday, batters have been plunked 0.64 times per game in day games and 0.61 times per game in night games. This years numbers are 0.70 for day games and 0.73 for night games, which are still above the 20 year average but well below the 2006 rate of 0.77 batters plunked per day game, and slightly below the 0.74 night game hbppg rate.