Fertilization and weed control work best when they're timed around the plant's growth cycle, not applied on a flat monthly schedule.
Early spring (pre-emergent season) is about stopping crabgrass and other summer weeds before they germinate — this window closes fast once soil temperatures warm up. Late spring and early summer applications focus on feeding the lawn through its most active growth period.
Late summer and fall broadleaf weed treatments tend to be the most effective of the year, since perennial weeds are pulling nutrients down into their root systems and carry the treatment down with them. Fall is also prime time for a final fertilization to build root strength before winter.
A season-long program beats one-off treatments because it keeps weeds from ever getting a foothold, rather than reacting after they've already spread.