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According to this link:
The following is required:
Getting More Data
Tableau requires at least five data points in the time series to estimate a trend, and enough data points for at least two seasons or one season plus five periods to estimate seasonality. For example, at least nine data points are required to estimate a model with a four quarter seasonal cycle (4 + 5), and at least 24 to estimate a model with a twelve month seasonal cycle (2 * 12).
If you turn on forecasting for a view that does not have enough data points to support a good forecast, Tableau can sometimes retrieve enough data points to produce a valid forecast by querying the datasource for a finer level of granularity:
- If your view contains fewer than nine years of data, by default, Tableau will query the data source for quarterly data, estimate a quarterly forecast, and aggregate to a yearly forecast to display in your view. If there are still not enough data points, Tableau will estimate a monthly forecast and return the aggregated yearly forecast to your view.
- If your view contains fewer than nine quarters of data, by default Tableau will estimate a monthly forecast and return the aggregated quarterly forecast results to your view.
- If your view contains fewer than nine weeks of data, by default, Tableau will estimate a daily forecast and return the aggregated weekly forecast results to your view.
These adjustments happen behind the scene and require no configuration. Tableau does not change the appearance of your visualization, and does not actually change your date value. However, the summary of the forecast time period in the Forecast Describe and Forecast Options dialog will reflect the actual granularity used.
Tableau can only get more data when the aggregation for the measure you are forecasting is SUM or COUNT. See Aggregations for information on available aggregation types and information on how to change the aggregation type.
If you switch your Date field to Month (or Quarter), you'll get a forecast at that level, but at the day level, it seems Tableau is unable to forecast with your data. I might suggested submitting your question to email@example.com to try and understand what is necessary to forecast at the Day level.
The most common reason that the forecast option is grayed out is that the date field is using the wrong date type. For example, instead of using Quarter Q2 use the lower option, Quarter Q2 2011.