Need help to create a relationship between one to many data files.

Yuiitsu 

You can create a date table in Power BI using DAX.

DateTable =
ADDCOLUMNS (
CALENDAR (DATE(2024, 1, 1), DATE(2024, 12, 31)),
"Year", YEAR([Date]),
"Month", MONTH([Date]),
"Day", DAY([Date]),
"MonthName", FORMAT([Date], "MMMM"),
"Quarter", "Q" & FORMAT([Date], "Q")
)

You can create a new table that extends the budget data to cover the necessary date ranges. 

ExtendedBudget =
VAR StartDate1 = DATE(2024, 1, 4)
VAR EndDate1 = DATE(2024, 9, 30)
VAR StartDate2 = DATE(2024, 10, 1)
VAR EndDate2 = DATE(2024, 12, 31)
RETURN
UNION (
SELECTCOLUMNS (
FILTER (
CROSSJOIN (
CALENDAR ( StartDate1, EndDate1 ),
BudgetTable
),
BudgetTable[Report Month] = StartDate1
),
"Date", [Date],
"Budget", BudgetTable[Budget]
),
SELECTCOLUMNS (
FILTER (
CROSSJOIN (
CALENDAR ( StartDate2, EndDate2 ),
BudgetTable
),
BudgetTable[Report Month] = StartDate2
),
"Date", [Date],
"Budget", BudgetTable[Budget]
)
)

Go to the "Model" view in Power BI.
Create a relationship between the Date column in the DateTable and the Date column in the ExtendedBudget table.
Create a relationship between the Date column in the DateTable and the Date column in the Forecast table.

Create measures to calculate the budget and forecast values for comparison. 

TotalBudget = SUM(ExtendedBudget[Budget])
TotalForecast = SUM(Forecast[ForecastValue])

Use the DateTable as the axis in your visualizations and plot the TotalBudget and TotalForecast measures to compare the values side by side.