A pollen calendar is used to show the peak pollen times for different types of plant pollens which cause allergic reactions in certain people.
Pollen calendar Wikipedia
A pollen calendar can be a very useful tool in forensic science, because it can be used to place the month, or week, or date of death. The use of pollen for criminal investigation purposes is called "Forensic palynology".
However, the use of a pollen calendar to set the date of death should be used with extreme caution, and only by a carefully trained expert witness. The CSI effect has pressured on some police officers and District Attorneys to provide pollen-based evidence, but "these appear to be of limited use in the forensic context where outcomes are scrutinised in court."